Acetate, labeled

Problem 29.5 Evidence for the role of acetate in fatty-acid biosynthesis comes from isotope-labeling experiments. If acetate labeled with 13C in the methyl group ( CFtyCC H) were incorporated into fatty acids, at what positions in the fatty-acid chain would you expect the, 3C label to appear ... 

The acetate labeling results clearly demonstrated a polyketide origin for the naphthoate fragment. This resulted in the hypothesis that the first enzyme-free intermediate in azinomycin biosynthesis would be naphthoate 102, with condensation to fonn a polyketone chain, reduction, cyclization, and dehydration/aromati-... 

Acetate, labeled at either the methyl or carboxyl position, was significantly incorporated into both citronellal and citral. 2-C14-Mevalonate was similarly well incorporated, but, in contrast, 1-C14-mevalonate produced only very slight radioactive labeling in the terpenes. The results of these experiments are summarized in Table I. 

FIGURE 23. 2D INADEQUATE spectrum of [l,2-13C2]acetate labeled rumbrin. Reproduced by permission of Japan Antibiotics Research Association from Reference 44... 

When compounds enriched in the heavy-carbon isotope 13C and the radioactive carbon isotopes nC and 14C became available about 60 years ago, they were soon put to use in tracing the pathway of carbon atoms through the citric acid cycle. One such experiment initiated the controversy over the role of citrate. Acetate labeled in the carboxyl group (designated [1-14C] acetate) was incubated aerobically with an animal tissue preparation. Acetate is enzymatically converted to acetyl-CoA in animal tissues, and the pathway of the... 

Cholesterol, like long-chain fatty acids, is made from acetyl-CoA, but the assembly plan is quite different. In early experiments, animals were fed acetate labeled with 14C in either the methyl carbon or the carboxyl carbon. The pattern of labeling in the cholesterol isolated from the two groups of animals (Fig. 21-32) provided the blueprint for working out the enzymatic steps in cholesterol biosynthesis. 

FIGURE 21-32 Origin of the carbon atoms of cholesterol. This can be deduced from tracer experiments with acetate labeled in the methyl carbon (black) or the carboxyl carbon (red). The individual rings in the fused-ring system are designated A through D. 

The D-mannopyranose pentaacetates were treated with stannic trichloride acetate (labeled with carbon14 in the carboxyl group) in chloroform solution, in the presence of stannic chloride. The rate at which radioactivity was introduced into the sugar acetate was followed. The exchange had been shown to be specific for the Cl-acetoxy group of the D-glucopyranose pentaacetates,33 and this was assumed in the present... 

Shihunine.—Preliminary results139 indicated that the orchid alkaloid shihunine (153) was derived from (152), an important intermediate in naphthoquinone biosynthesis.140 Further details are now available in a full paper.141 The intact incorporation of (152) is affirmed by the observation that (152), labelled with 13C at C-l, was an efficient and specific precursor for (153). [l-14C]Acetate was examined as a shihunine precursor and was found only to label C-5. This is consistent with the expected formation of (152) from shikimic acid (144) and a-ketoglutarate (151),140 the latter gaining acetate label in its carboxy-groups through the tricarboxylic acid cycle. 

The presence of two heteroatoms complicates the mechanism. In fact, propyl acetate yields protonated acetic acid as a base peak, whereas methylbutyrate yields an (RCOOH + H)+ peak that is hardly detectable the proton is derived from the alcohol. Experiments on propyl acetate labelled with deuterium at various positions on the propyl chain yield the following percentages concerning the origin of the proton during the rearrangement. This distribution seems fairly statistical ... 

Lemieux and Brice have measured the rate of exchange (in chloroform) of a large number of 1,2-trans sugar acetates with stannic trichloride acetate labeled with C. Their results are given in Table XVII. The low reactivities of the acetates of 8-D-ribose, a-D-lyxose, 8-D-allose, and a-D-mannose (rela-... 

Figure 8 Polyketides of the chalcone super family of chalcone and stilbene (The illustrated [ 2]acetate labeling is extrapolated from related incorporation studies)...

Figure 26.6. Labeling of Cholesterol. The results of isotope-labeling experiments reveal the source of carbon atoms in cholesterol synthesized from acetate labeled in its methyl group (blue) or carboxylate atom (red).

Chatoglobosin A (58) and the related compounds are mycotoxins produced by various iungi such as Chaetomium sp. These mycotoxins are acutely toxic to mammals [46]. Chaetoglobosin O was isolated from Cylindrocladium Jloridanum causal fungus of black rot disease to alfalfa [47]. Biosynthetic studies using the cheatoglobosin-produc-ed strain Chaetomium subqffine revealed that the incorporation of the precursor [1- C, 02]-, H3]-acetate, labeled methionin and... 

The use of i3C2-acetate labelling to detect the involvement of symmetrical intermediates has found extensive use in biosynthetic studies. Thus, as shown in Scheme 4, incorporation of [1, 2- 2] acetate into ravenelin (7) in cultures of Helminthosporium ravenelii resulted in the observation of a randomisation of labelling in ring-C which confirmed the predicted involvement of a symmetrical benzophenone intermediate (6), itself derived from cleavage of an octaketide-derived anthraquinone, presumably helminthosporin (5) [12]. 

In marked contrast to ravenelin and tajixanthone, no randomisation of [1, 2- 2] acetate labelling was observed on incorporation into sterigmatocystin (20) in Aspergillus variecolor, ruling out the involvement of a proposed symmetrical benzophenone intermediate [17]. 

Detoxification. A ten minute incubation of both trladlmenol resistant and sensitive U. avenae strains In C1 acetate, labelled 4,4 dimethyl sterols equally In both strains, when trladlmenol was present. After further incubation in the absence of radio-isotope, methyl sterols were no longer labelled In the resistant strain, suggesting that rapid turnover... 

The biosynthetic pathway to prostaglandins leads also to a class of physiologically potent substances known as prostacyclins. Which carbon atoms of the prostacyclin shown here would you expect to be enriched in if it were biosynthesized from acetate labeled with in its methyl group ... 

The origin of the C-2 unit in anhalonidine (59) and pellotine (61) has received attention. [2-Acetate gave pellotine with an approximately equal distribution of the label over carbons 1 and 9, whilst [l- C]acetate labelled C-1 to the extent of 59% of the total activity and C-9, 26%. Thus, it appears that although acetate is able to act as an efficient precursor, it is undergoing extensive degradation in vivo. Pyruvate has been shown to be a more effective precursor of the two-carbon unit than acetate. Administration of [3- C]pyruvate to L. williamsii gave radioactive anhalonidine (59) with activity spread over both C-1 and C-9 but with a major portion confined to C-9. 

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