Randomization of label

It is evident that the studies with tritiated proline do not permit definite conclusions about the mechanism of hydroxylation further information is needed about the labeling of the proline used and possible isotope effects. It is probable that there is some randomization of label during tritiation of 3,4-dehydroproline (40), and apparently also some racemization. Thus, we have observed that the tritiated proline preparations contain 10 to 20% D-proline and that oxidation by D-amino acid oxidase releases significant amounts of tritiated water. 

Much interest has been shown in the biosynthesis of insect juvenile hormones (62 R1, R2 = Me or Et). In adult male moths, [l-14C]propionate was specifically incorporated into juvenile hormone I [JH-1, (62 R1 = R2 = Et)], and tracer was only found at, and equally distributed between, C-7 and C-ll.90 Application of [2-14C]-and [3-14C]-propionate led to extensive randomization of label, which suggests that C-2 and C-3 formed in propionate catabolism can be re-used as smaller fragments, whilst C-l is either removed from propionate in a metabolically active form or is highly diluted. Ternary complexes of brain, corpora cardiaca, and corpora allata from the tobacco budworm Heliothis virescens produced labelled JH-I and JH-II (62 R1 = Et, R2 = Me) when incubated with L-[Me-14C]methionine or sodium [l-l4C]propionate.91 Partial degradation of the juvenile hormones showed that in JH-I portions a and /3 (62) had incorporated one atom of tracer from each propionate, whereas fraction y was unlabelled, and in JH-II only fraction a was... 

Two metabolic patterns are discernible from the results. Carbon atoms 2, 1, and 7 of shikimate (VI) are derived almost equally from G-1,6, G-2,5, and G-3,4, respectively. In the Embden-Meyerhof pathway of hexose metabolism (see Fig. 2), D-fructose 1,6-diphosphate is cleaved to 1,3-dihydroxy-2-propanone phosphate (G-1,2,3) and D-glycerose 3-phosphate (G-4,5,6), and the two trioses are interconverted by triose phosphate isomerase. The observed randomization of label between Cl and C6, C2 and C5, and C3 and C4 of hexose therefore implies that C2, Cl, and C7 of shikimate are derived from a 3-carbon intermediate of glycolysis. The small but significant preponderance of G-6 over G-1, of G-5 over G-2, and, presumably, of G-4 over G-3, can be explained by recent observations that, in the aldolase cleavage of D-fructose 1,6-diphosphate, the 1,3-dihy-... 

Prockop and others (258, 259) prepared a sample of [3,4- H2]proline 252, Hg = Ht, = H, by catalytic tritiation of 3, 4-dehydroproline, and while acknowledging that some randomization of label might occur in this process. 

The incorporation C. autumnale) of 0-methylandrocymbine (CXVII R = CH3, tritium label in the OCH3 at carbon 2) and CXVIII (i C at carbon 6) into colchicine (15 and 10% incorporation, respectively) with no randomization of label indicated the general correctness of the proposed scheme. Multiple labeling experiments with synthetic CXVIII have confirmed many details of the biosynthesis. CXIX was incorporated into colchicine with no change in the ratio, proving that... 

Uesato, 1986). Randomization of label from [2- " C]MVA between C-3 and C-11 usually was observed in iridoid and secoiridoid glycosides and in indole alkaloids (Inouye and Uesato, 1986). 

The degradation scheme did not allow the location of all the radioactivity in deoxystreptamine. However, C-4, C-5, and C-6 (corresponding to C-3, C-4, and C-5 of the hexoses) contained only 15 % of the radioactivity, corresponding ss in neosamine B to about 30% randomization of label. The parallel results would be in agreement with a common precursor for deoxystreptamine and neosamine B. 

C(15)-C(16) and C(6)-C(7). The absence of couplings elsewhere in the spectrum indicated that there was negligible randomization of label into the hexanoate residue via [l,2- C]acetate. 

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