Archives of

Micrnbinlngy

Arch. Microbiol. 118, 7-12 (1978)

9 by Springer-Verlag 1978

Incorporation of Labeled Small Molecules into Rubratoxin C. Obi Emeh and Elmer H. M a r t h Department of Food Science and the Food Research Institute, University of Wisconsin-Madison, Madison, WI 53706, U.S.A.

Abstract. A sterile glucose-mineral salts broth was inoculated with conidia o f Penicillium rubrum P-13 and P-3290. Radiolabeled c o m p o u n d s were added to some cultures, these being incubated quiescently at 28 ~ C for 14days. Other stationary cultures were grown for 21 days, received labeled compounds, and were then grown for 5 more days. The remaining cultures were inoculated with 72-h-old mycelial pellets, received labeled materials and were incubated with shaking for 60 h. R u b r a t o x i n was resolved by thin-layer c h r o m a tography. Labeled [ll~C]acetate, [1,514C]citrate, [2~4C]malonate, [114C]glucose, [U14C]glucose or [1 ~4C]hexanoate were incorporated into rubratoxins A and B by P. rubrum 3290 and into rubratoxin B by P. rubrum 13. I n c o r p o r a t i o n o f [114C]acetate and [2agC]malonate increased when exogenous unlabeled acetate, malonate, pyruvate, or phosphoenol-pyruvate was added. Acetate incorporation was influenced by cultural conditions, attaining m a x i m u m amounts in quiescent cultures which received labeled acetate after 21 days o f incubation. Acetate incorporation in shake cultures was enhanced by reduced nicotinamide adenine dinucleotide phosphate ( N A D P H ) and by unlabeled exogenous citrate.

Key words: R u b r a t o x i n -

Mycotoxin -

Penicillia -

PeniciIlium rubrum.

Metabolism-

Mold -

Rubratoxins are metabolites o f Penicillium rubrum and Penicillium purpurogenum Stoll (Moss, 1971; Natori et al., 1970) and have been evaluated for their toxicity to laboratory and domestic animals (Hayes and Wilson, 1970). Their structure and physicochemical properties have been determined (Moss, 1971 ; Moss et al., 1968). Abbreviations. GMS = glucose-mineral salts; RCM = replacement

culture medium; TCA = tricarboxylic acid; PEP = phosphoenolpyruvate; RIC = relative isotopic content; PI = percent incorporation

A l t h o u g h biosynthesis o f related c o m p o u n d s such as glaucanic and glauconic acid has been described (Bloomer et al., 1965), only limited information on biosynthesis o f rubratoxins is available. Hayes (1972) described tissue-distribution of radioactivity from r u b r a t o x i n B labeled with [14C]acetate. His results suggested the possibility o f incorporating related small molecules into rubratoxin. Since (1) biological activities o f mycotoxins including rubratoxin are usually evaluated with the appropriately labeled c o m p o u n d (Hayes and Wilson, 1970), and (2) additional information on metabolic development by P. rubrum is needed (Emeh and Marth, 1977), we ascertained h o w several radiolabeled small molecules are incorporated into rubratoxins.

Materials and Methods Organisms. Test organisms were Penicillium rubrum P-13 (NRRL11785) (from B. J. Wilson of Vanderbilt University, Nashville, Tenn., U.S.A.) and P. rubrum P-3290 (from the Northern Regional Research Center, U.S. Department of Agriculture, Peoria, Ill., U.S.A.). Culture Media. The glucose-mineral salts broth (GMS) was that of Emeh and Marth (1977). Glucose, nitrogen sources, and mineral supplements were sterilized separately by autoclaving to avoid caramelization and to minimize precipitation. Medium components were combined on cooling and the pH was adjusted to 6.0 with 2 N NaOH. The replacement culture medium (RCM) contained (g/l): glucose, 15; KH2PO4, 0.75; MgSO4.7H20 , 0.5; KC1, 0.5; and the same trace elements as in GMS broth. Radioactive Materials. [la4C]Sodium acetate (specific activity

57 mCi/mmol), [2a4C]malonicacid (specific activity 5.6 mCi/mmol), and [114C]glucose(specific activity 6.37 mCi/mmol) were purchased from New England Nuclear Corporation, Boston, Mass., U.S.A. [UlgC]Glucose (specific activity 2.9 mCi/mmol), [1,5~4C]citric acid (specific activity 1.9 mCi/mmol, and [114C]n-hexanoicacid (specific activity 23.6mCi/mmol) were purchased from Amersham/Searle Corporation, Arlington Heights, Ill., U.S.A. All tracers, 10 mCi/10 ml of medium in 50-ml Erlenmeyer flasks, or 40 ~Ci/50 ml of medium in 300-ml Erlenmeyer flasks, were diluted with appropriate unlabeled compounds, the pH of solutions being adjusted to 6.0 with 2 N HC1.

0302-8933/78/0118/0007/$01.20

8

Spore Suspensions. Stock cultures of P. rubrum P-I 3 and P-3290 were maintained on corn slants and stored at 5~ after the mold had grown. Conidia were obtained by shaking such corn slants of P. rubrum with sterile water containing 0.006% Leconal (Arthur Thomas, Philadelphia, Pa.). A suspension of conidia (lml with 2 x 105 spores) was used to inoculate 50-ml portions of sterile GMS broth in 300-ml Erlenmeyer flasks. Duplicate samples of cultures of either P. rubrum strain were treated with 40 ~Ci of appropriate radiolabeled compounds and all cultures were grown quiescently at 28~C for 14days. Mycelial Cultures. Inoculated media prepared as just described were incubated quiescently at 28~C for 21 days. [114C]Acetate, [214C]malonate, [114C]glucose,or [U14C]glucose(40 gCi/flask) were added and cultures were held at 28~C for 5 more days. Replacement Cultures. SterileGMS broth (100ml/500-mlErlenmeyer flask) was inoculated with a suspension of conidia (106/ml) of P. rubrum P-13 or P-3290. Cultures were incubated with shaking at 30~C on a reciprocal shaker (New Brunswick Scientific Co., New Brunswick, N.J., U.S.A.) at 100 strokes/min for the first 24h and at 200 strokes/min thereafter. After 72 h of incubation, all cultures were harvested. The mycelium was recovered by filtering the culture through cheesecloth at reduced pressure. Then the mycelium was washed with distilled water, and a weighed amount (0.6 g/flask) was used to inoculate a 10-ml quantity of RCM in a 50-ml Erlenmeyer flask. [114C]Acetate, [214C]malonate,[114C]glucose,[U14C]glucose, [ll4C]hexanoate, or [1,514C]citrate (101aCi/flask) was added to P. rubrum cultures which were then incubated for 60 h at 30~ with shaking at 200strokes/min. Cultures were harvested and labeled rubratoxins were recovered from culture filtrates. Extraction of Rubratoxins. Filtrates from stationary cultures of P. rubrum (50 ml/300-ml Erlenmeyer flask) were concentrated by flash evaporation (Rinco Instruments, Chicago, Ill., U.S.A.) to 10ml Filtrates from shake cultures were mixed with washings(up to 50 ml) of mycelial pellets. Mixtures were then concentrated (Rinco flash evaporator) to 10ml. All samples were extracted twice with acetone: ethanol (5 : 1, vol/vol). The resulting extract (100 ml/sample) was concentrated (Rinco flash evaporator) to 20 ml and then its pH was adjusted to 1.5 with 6 N HCI. The extract was then mixed with diethyl ether (AR grade), held overnight, and evaporated to dryness. Resolution and Purification of Rubratoxins. The brown residue from the extraction procedure was dissolved in 1ml of acetone and then spotted on a thin-layer chromatographic plate (20 x 20 cm) having a l-mm-deep layer of silica gel HFz54+366 (Kensington Corporation, Berkeley, Cal., U.S.A.). Chromatograms were developed and examined as described earlier (Emeh and Marth, 1977). A standard (from M. O. Moss, Surrey, England) was also spotted on each chromatoplate. Bands presumed to contain rubratoxins were scraped from chromatographic plates and acetone was used to elute rubratoxins from the silicagel. Rubratoxins were purified by repeated (four times) thin-layer chromatography using plates with a 0.5-ram-deeplayer of silica gel HFz54+366. Confirmation of rubratoxins was done with a scanning spectrophotometer (ACTA III, BeckmanInstruments, Inc., Fullerton, Cal., U.S.A.) at 251 and 252nm and at the molar extinction coefficientsrecommended by Moss (1971). Concentration of toxin was calculated from absorbance readings based on a standard curve and taking into account the dilutions involved. A portion of the toxin recovered from thin-layer chromatoplates having the 0.5-mm-deeplayer of silicagel was eluted with acetonitrile and was used to determine radioactivity. The amount of radioactivity was measured as described earlier (Emeh and Marth, 1977). Counts (disintegrations per min, dpm) were corrected for absolute specific activities by using internal standards (Chase and Rabinowitz, 1962). Efficiency of Rubratoxin Synthesis. The efficiencyof incorporation of radiolabeled compounds into rubratoxin was determined as percent

Arch. Microbiol., Vol. 118 (1978) incorporation (PI) and as relative isotopic conten (RIC). These values were obtained as described by Hsieh and Mateles (1971).

Results and Discussion I n v o l v e m e n t of fatty acyl derivatives in biosynthesis of fungal metabolites was predicted over 50 years ago b u t it was only d u r i n g the last 20 years that the acetate hypothesis was applied directly to synthesis o f fungal secondary metabolites ( T a n e n b a u m , 1965). According to this theory, which has been proposed for r u b r a t o x i n biosynthesis by Moss (1971), acetate is j o i n e d to m a l o n a t e in a " h e a d to tail" c o n d e n s a t i o n with sequential a d d i t i o n of m a l o n a t e units a n d a c c o m p a n y i n g d e c a r b o x y l a t i o n to form a starter molecule (Bentley a n d Campbell, 1971; G u r r a n d James, 1971; T u r n e r , 1971 ; Vagelos, 1974). C h a i n initiation a n d elongation, followed by d i m e r i z a t i o n a n d cyclization results in f o r m a t i o n of rubratoxin. I n c o r p o r a t i o n of labeled small molecules into r u b r a t o x i n is examined in the r e m a i n d e r o f this report.

Rubratoxin Synthesis in Quiescent Cultures [114C]Acetate, [214C]malonate, [ll4C]glucose, a n d [uX4C]glucose were i n c o r p o r a t e d into r u b r a t o x i n s A a n d B by Penicillium rubrum 3290 a n d into r u b r a t o x i n B by P. rubrum 13 (Table 1). I n all instances, quiescent cultures held for 21 days before addition o f labeled material i n c o r p o r a t e d more radioactivity into toxin t h a n did cultures which received labeled precursors at the b e g i n n i n g of the experiment (i.e., cultures held for 14 days). P. rubrurn 13 i n c o r p o r a t e d almost three times as m u c h [la4C]acetate into r u b r a t o x i n when cultures were held for 26 rather t h a n 14 days. The same was true for m a l o n a t e , i n c o r p o r a t i o n being more in 26-day-old cultures t h a n in those held for 14days. Glucose inc o r p o r a t i o n was a b o u t 50 % of the a m o u n t recorded for acetate. Acetate i n c o r p o r a t i o n into toxin is diluted by e n d o g e n o u s n o n l a b e l e d acetate or acetate formed from other sources. I n c o r p o r a t i o n of b o t h acetate a n d m a l o n a t e provides evidence to s u p p o r t the theory that the p o l y m a l o n a t e p a t h w a y is involved in biosynthesis of r u b r a t o x i n ( T a n e n b a u m , 1965; G u p t a et al., 1974; Bu'Lock, 1965). The lower i n c o r p o r a t i o n efficiency o b t a i n e d in cultures held for 14 rather t h a n 26 days m a y have resulted because labeled precursors were introduced before f o r m a t i o n of r u b r a t o x i n . Thus, radioactivity in these precursors m a y have d i s s i p a t e d , intermediates being washed out as fresh material was introduced. A l t h o u g h toxin ultimately c o n t a i n e d more radioactivity t h a n residual acetate, unspecific incorpor a t i o n into other metabolites results in a n overall low i n c o r p o r a t i o n efficiency a n d low isotopic c o n t e n t (Table 1). Less radiolabeled glucose than acetate was i n c o r p o r a t e d into r u b r a t o x i n p r o b a b l y because iso-

C. O. E m e h a n d E. H . M a r t h : I n c o r p o r a t i o n o f L a b e l e d S m a l l M o l e c u l e s i n t o R u b r a t o x i n

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topes of greater mass react less with unlabeled endogenous materials and fungal metabolites than do those of smaller molecular weight (Bu'Lock, 1965; Conn and Stumpf, 1972; Dawes, 1972). Also, since glucose gives rise to several intermediates during mold catabolism, the accompanying exchange reactions will reduce the carbon fragments incorporated into toxin (Conn and Stumpf, 1972). Rubratoxin Synthesis in Shake Cultures

>

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Incorporation of various labeled small molecules is shown by data in Table 2. [114C]Acetate, [114C]glucose, and [U14C]glucose were incorporated into rubratoxin A by P. rubrum P-3290; both P. rubrum P-13 and P-3290 incorporated [21~C]malonate, [114C]hexanoate, [1,5~4C]citrate, and labeled acetate and glucose into rubratoxin B. Radiolabeled materials were incorporated more efficiently into rubratoxin B than into A. As is shown by data in Table 1, acetate was the preferred substrate, although [U14C]glucose incorporation into rubratoxin by P. rubrum (P-13 and P-3290) was almost equal to that of acetate. Acetate, when used as a sole source of carbon, has been incorporated directly into aflatoxin (Hsieh and Mateles, 1971) and has been established as a key intermediate in initiating secondary metabolism in fungi. In the course of metabolic phasing (primary as well as secondary), acetate may be converted to orsellinate through condensation of polyacetate units (Bentley and Campbell, 1971; Turner, 1971). During fatty acid formation acetyl-CoA and malonyl-CoA are the "starter" molecules. When these react together, CO2 is released and aceto-acetyl CoA and its derivatives are formed (Bentley and Campbell, 1971; Bu'Lock, 1965). Subsequent recycling may occur until a C16-C18 carbon chain is obtained, when fatty acids may be cleaved off. Similar events may occur in rubratoxin biosynthesis because from comparative studies ofglaucanic and glauconic acid formation (Bloomer et al., 1965; Moppett and Sutherland, 1966), Moss et al. (1968) have suggested the following. A C9 intermediate may be derived by coupling of a hexanoic acid derivative and oxaloacetic acid. Similarly, rubratoxins may be produced by head-to-tail, head-to-tail coupling of two C13 units. However, because of smaller pools of malonyl CoA incorporated into the fatty acid chain, [2a4C]acetate yields a higher incorporation efficiency (Bu'Lock, 1965). For longer chain intermediates (C13 for rubratoxin) the malonate-derived chain shows nonuniform labelling. This may account for lower incorporation efficiencies by cultures exposed to labeled malonate. The Ca-carbon of [1 ~4C]glucose is released as C4; C 5 and C 6 yield glyceraldehyde-3-phosphate, the carboxyl group of acetate being derived from C4. Under

10

Arch. Microbiol., Vol. 118 (1978)

Table 2. Synthesis of rubratoxin by replacement cultures of Penicillium rubrum P-13 and P-3290 grown in a glucose-mineral salts broth with shaking at 30~ for 60h" Radiolabeled material added (10 ~tCi/flask b)

[114C]Acetate [214C]Malonate [1 t4C]Glucose [Ul~C]Glucose [114C]Hexanoate [t,5~4C]Citrate

Strain of Penicillium rubrum P-13

P-3290

Rubratoxin B

Rubratoxin A

Rubratoxin B

retool/ flask (• 3)

PI ~

RIC d

retool/ flask (•

PI

RIC

retool/ flask (•

PI

RIC

0.13 0.15 0.36 0.34 0.15 0.06

2.5 1.8 1.7 1.9 0.8 2.3

0.19 0.12 0.14 0.19 0.023 0.18

0.22 0.20 0.34 -

3.8 1.6 1.4 -

0.17 0.11 0.14 -

0.10 0.14 0.82 0.54 0.22 0.03

2.2 2.5 3.8 2.3 1.0 1.9

0.22 0.18 0.16 0.15 0.019 0.24

a Rubratoxin yields (mmol, PI, RIC) are averages of duplicate values. Toxin yields varied between.1 and 5 ~o b 10 ml of replacement culture medium in 50-ml Erlenmeyer flask ~ PI = radioactivity of radiolabeled compound into toxin as percentage of total in material d RIC = relative isotopic content, a measure of the quantity of radiolabeled compound (retool) incorporated into rubratoxin divided by the yield of rubratoxin (mmol). PI, RIC varied between 3 and 9

aerobic conditions, terminal oxidation functions as part of the control of the tricarboxylic acid (TCA) cycle. Pyruvate is oxidized to acetyl CoA and during glycolysis C 1 and C 6 glucose carbons are equally distributed as both are incorporated into methyl positions in intermediates. This will result in more incorporation of [D-U14C]glucose. When the pentose phosphate cycle is in operation, radioactivity is unequally distributed. Thus C~ yields CO 2 and C 6 yields CO 2 upon completion of the TCA cycle (Dawes, 1972). In the absence of endogenous dilution of radioactivity, all the carbon atoms in [U14C]glucose should have the same specific activity as the substrate (acetate) carbon (Chase and Rabinowitz, 1962; Dawes, 1972). However, because of the many exchange reactions occurring between Embden-Meyerhof-Hexose Monophosphate-TCA cycles [since many molds, including Penicillium sp., probably use more than one pathway simultaneously (Dawes, 1972)], the pattern of [U~C]glucose incorporation is similar to that of [114C]glucose. Further, the rate of dilution of labeled precursors of fungal metabolites is inversely proportional to their degree of relatedness to the metabolite. Thus the further away the product is from the precursor, the greater is the dilution of radioactivity (Bu'Lock, 1965; Conn and Stumpf, 1972; Segel, 1968). Consequently, a lower incorporation efficiency was obtained from labeled glucose, citrate, and hexanoate. In addition, less specificity of incorporation of citrate and hexanoate (namely degradation to C 2 units) may account for the low incorporation of these labeled compounds into rubratoxin (Moppett and Sutherland, 1966).

Incorporation of Labeled Compounds into Rubratoxin in the Presence of Unlabeled Precursors The effect of unlabeled exogenous materials on acetate incorporation into rubratoxin is shown by data in Table3. At low concentrations, 10-4M/flask, unlabeled acetate, malonate, and phosphoenol-pyruvate (PEP) enhanced [114C]acetate incorporation into rubratoxinB. Radiolabeled acetate was incorporated into rubratoxinA only when PEP was added to P. rubrum cultures. At a concentration of 10 -3 M/flask, acetate and malonate caused incorporation of less labeled acetate into rubratoxin, although citrate caused an increase in acetate incorporation into rubratoxin B in P. rubrum P-3290. Pyruvate (10-3 M/flask) caused a 50~ increase in acetate incorporation into rubratoxin B by P. rubrum P-13, it also caused a slight increase in incorporation of labeled acetate into rubratoxin A and a reduction in rubratoxin RIC in P. rubrum P-3290. At a concentration of 10-2 M/flask of both unlabeled acetate and malonate, no labeled acetate was incorporated into rubratoxin, probably because of complete dilution of the radioactivity in labeled acetate. Dilution techniques are used to obtain evidence of synthesis and precursor product relationships between known compounds. The labeled material introduced is trapped during mold metabolism, thus labeled acetate would be diluted by low concentrations of unlabeled acetate, converted to malonyl CoA, and the starter molecule thus generated eventually would be converted to rubratoxin. Therefore, enhance-

C. O. Emeh and E. H. M a r t h : Incorporation of Labeled Small Molecules into Rubratoxin

1

Table 3. Effect of adding unlabeled compounds on incorporation of [1 ~4C]acetate into rubratoxin by replacement cultures ofPenicillium rubrum P-13 and P-3290 grown in a glucose-mineral saIts broth with shaking at 30~ for 60h" Unlabeled compound added (exogenous)

Concentration of unlabeled material (retool/ flask) ~

Strain of Penicillium rubrum P-13

P-3290

Rubratoxin B

Rubratoxin A

(mmol)

PI ~

RIC ~

(mmol)

xlO -3

Control b Acetate

Malouate

0.l 1.0 10.0 0.1 1.0 10.0

Phosphoenol pyruvate pyruvate citrate " b ~ d e = of t

0.01 1.0 1.0

Rubratoxin B

PI ~

RIC ~

(mmol) x 10 -3

PI e

RICr

3.8 -

0.18 -

0.12 d 0.13 0.12 f 0.08 0.08 f

2.8 3.8 2.3 -2.5 2.1

0.23 0.28 0.19 -0.31 0.25

2.5 0.85

0.16 0.06

0.17 0.14 0.18

3.4 2.9 2.1

0.20 0.21 0.13

xlO -3

0.15 a 0.09 0.13 -0.11 0.16

3.6 4.2 2.6 f 2.7 2.4 f

0.24 0.46 0.20 . 0.25 0.15

0.17 0.08 0.19

3.0 2.8 2.9

0.18 0.35 0.14

0.21 d .

.

.

0.15 0.14

Rubratoxin content of samples expressed as averages of duplicate samples Radiolabeled acetate ([114Clacetate 10 ~LCi/flask) 10 ml of replacement culture medium in 50-ml Erlenmeyer flask Yield of toxin: (retool/flask). Values varied between 5 and 13 PI = a m o u n t of radioactivity of c o m p o u n d incorporated into rubratoxin as a percentage of the total a m o u n t of compound used; R I C relative isotope content, a value obtained by dividing the quantity (mmol) ofradiolabeled c o m p o u n d incorporated into rubratoxin by the yield rubratoxin (retool). Values varied between 3 and 16 Rubratoxin recovered was not labeled. Mevalonate, leucine were not incorporated

Table 4. Effect of exogenous unlabeled reduced nicotinamide adenine dinucleotide phosphate (NADPH2) on incorporation of [114C]acetate into rubratoxin by replacement cultures of Penieillium rubrum P-13 and P-3290 grown with shaking at 30~ for 60 h a Strain of Penicillium rubrum

Exogenous unlabeled material

P-13

P-3290

Rubratoxin B

Rubratoxin A

Yield ~ (mmol) •

Control b N A D P H 2 , 1 mg NADPH2, 10 mg

pI d

RIC d

Yield c (mmol)

-3

0.21 0.14 0.16

•

4.1 3.8 5.6

0.20 0.28 0.35

Rubratoxin B

pI d

RIC d

Yield ~ (mmol)

3

pI d

RIC d

2.8 2.9 3.1

0.23 0.27 0.39

•

0.2t 0.112 0.089

3.8 2.9 2.7

0.18 0.26 0.31

0.12 0.11 0.079

Toxin content of samples is expressed as averages of two experimental (duplicate) samples b Glucose mineral salts broth 10ml/flask (replacement culture medium containing 10~tCi [i i4C]acetate/flask and 10 4 M unlabeled acetate) c Moles of rubratoxin/10 ml medium in a 50-ml Erlenmeyer flask. Values varied between 4 and 10 d PI = a m o u n t of radioactivity of compound incorporated into rubratoxin as a percentage of the total; R I C = relative isotope content, a value obtained by dividing the a m o u n t of radiolabeled substance (mmoI) incorporated into rubratoxin by the a m o u n t of toxin recovered (mmol). Values varied for both PI and RIC between 2 and 5

ment

of acetate

increment

incorporation

in acetyl CoA

and the concomitant

may

(oxidative)

compensation

result

1967).

an

decarboxylation,

for the requirement

of acetyl CoA used in fatty acid biosynthesis 1974; Lynen,

from

(Vagelos,

Effect of Reduced Nicotinamide Adenine Dinucleotide Phosphate ( N A D P H ) on Acetate Incorporation into Rubratoxin The

effect of NADPH

acetate

into

rubratoxin

on

incorporation

was influenced

of [114C]by NADPH

12

concentration, strain of fungus, and type of toxin (Table 4). At a concentration of 1 rag/flask, NADPH caused a slight reduction in acetate incorporation (PI) into rubratoxin B by P. rubrum 13; simultaneously, a 40 ~ increase in RIC was recorded. When the NADPH concentration was 10rag/flask, the PI was enhanced almost 33 ~ and the RIC by 80 ~. In P. rubrum 3290, NADPH caused only a slight increase in incorporation efficiency measured as PI. The increase in RIC was more substantial- almost 50 ~ and 67 ~o (produced by addition of I and 10 mg of NADPH) in rubratoxin B. The source of reducing power for biosynthesis is NADPH and this pyridine nucleotide is generated principally by the pentose phosphate (HMP) cycle (Cohen, 1968). Normally, aeration through the entire growth period would cause partial oxidation of the NADPH through transdehydrogenation to form ATP (Bentley and Campbell, 1971; Bu'Lock, 1965; Cohen, 1968). We added NADPH to the incorporation system since earlier results of low toxin yield in vigorously aerated (agitated) cultures (Emeh and Marth, 1976) suggested that much of the NADPH produced in such cultures were channeled into metabolites other than rubratoxins (Maguigan and Walker, 1940; Turner, 1971). It seems clear that a much higher efficiency for acetate incorporation would be expected than was obtained but it is quite likely that more than one pathway was in operation during mold growth (Dawes, 1972). Thus losses may have resulted from nonspecific incorporation of acetate and malonate into intermediates other than rubratoxin (Turner, 1971). The overall low incorporation efficiency may have resulted from incorporation of labeled precursors into lipid as has been reported earlier (Tanenbaum, 1965; Bentley and Campbell, 1971 ; Turner, 1971 ; Bu'Lock et al., 1965). Lipid formation and rubratoxin synthesis have a similar initial p a t h w a y - a buildup of acetate and malonate (Moss, 1971; Bentley and Campbell, 1971). It is also possible that the same pathway operates in the initial chain elongation for both lipid and rubratoxin formation (Bentley and Campbell, 1971). Acknowledgments. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, and by Public Health Service Grant FD000143 fi'om the U.S. Food and Drug Administration.

References Bentley, R., Campbell, I. M.: Secondary metabolism in fungi. In: Comprehensive biochemistry, Vol. 21 (M. Florkin, E. Stotz, eds.), pp. 415-489. New York: American Elsevier 1971 Bloomer, J. L., Moppett, C. E., Sutherland, J. K. : The biosynthesis of glauconic acid. Chem. Commun. 21, 619-621 (1965)

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Received January 4, 1978