Jesse C. Rabinowitz

Vitamin B6 Group. XV. Urinary Excretion of Pyridoxal, Pyridoxamine, Pyridoxine, and 4-Pyridoxic Acid in Human Subjects.*:

Summary The known metabolic products of vitamin B6—pyridoxal, pyridoxamine, pyridoxine and pyridoxic acid—were measured in normal human urine and in the urine of human subjects each fed one of the 3 forms of the vitamin. The chief product found, regardless of the form fed, was pyridoxic acid. Pyridoxal gave rise to significantly higher amounts of this product than did pyridoxine or pyridoxamine. No evidence could be obtained showing the conversion of pyridoxal or pyridoxamine to pyridoxine. When pyridoxal or pyridoxine was fed, the chief form in which the vitamin oc- curred in the urine was the form fed. However, when pyridoxamine was fed both pyridoxal and pyridoxamine were excreted in approximately equal amounts. Ingestion of pyridoxine also greatly increased the amount of pyridoxal and pyridoxamine excreted. The excretion of all products was very rapid. The largest amounts of each of the compounds were found in samples collected 2 and 5 hours after ingestion of the dose. The levels of pyridoxic acid returned to normal values after 12 hours, while the vitamin levels had returned to normal within 8 hours. The amount of the dose recovered varied with the form fed. The highest recovery, 70%, was obtained when pyridoxal was fed; 45% of the pyridoxine was recovered, while only 31% of the pyridoxamine could be recovered. Together with published data which indicate that complete absorption of large doses of vitamin B6 occurs, these findings suggest that a large proportion of the vitamin B6 was converted to products still unknown.

The nature of the requirement of Saccharomyces carlsbergensis for vitamin B6

Abstract Saccharomyces carlsbergensis 4228, an organism widely used for determination of vitamin B6, grows well without this vitamin if thiamine is also omitted from the basal medium, and an inoculum grown in a thiamine-low medium is used. Thiamine inhibits growth when added to such a medium. The thiazole moiety of thiamine, but not the pyrimidine, is also inhibitory, but less so than thiamine itself. Growth inhibition by thiamine is prevented by vitamin B6. At low concentrations of thiamine, the amount of vitamin B6 required for growth increases with the thiamine concentration; at concentrations of thiamine above 1 μg./10 ml. the vitamin B6 requirement for growth remains essentially constant. Since these higher concentrations of thiamine have been used in methods that utilize this organism for determination of vitamin B6 (1,2), the validity of these methods is confirmed. In the presence of thiamine, growth was also permitted by additions of the thiamine antagonist, neopyrithiamine. In this case, however, the relationship was fully competitive; i.e., the amount of neopyrithiamine required for growth increased regularly with the thiamine concentration. At concentrations considerably higher than those required for growth, neopyrithiamine again inhibited growth, and this inhibition was prevented by an increase in the thiamine concentration. Thus neopyrithiamine acts by lowering the effective thiamine concentration to subinhibitory levels; if excessive amounts are used, it prevents essential metabolic functions of thiamine and itself becomes toxic. The mechanism by which vitamin B6 prevents thiamine toxicity is not known. The appearance of a requirement for certain growth factors because of inhibitory effects of other metabolically important compounds, rather than because of an intrinsic inability of the organism to synthesize the growth factor, may be much more common than the few recorded instances of this phenomenon indicate.

Vitamin B6 antagonists and growth of microorganisms. I. 4-Desoxypyridoxine

Abstract 4-Desoxypyridoxine is relatively ineffective as an antagonist of vitamin B 6 for organisms that grow in the absence of the latter vitamin, and for many lactic acid bacteria that require the vitamin. It is an effective antagonist of vitamin B 6 for certain yeasts and molds that require an external source of vitamin B 6 for growth. It is shown that the same organism may be sensitive to the inhibitor when dependent upon an external supply of vitamin B 6 for growth, but insensitive when cultured under conditions that permit it to synthesize this vitamin. The comparative effectiveness of various forms of vitamin B 6 in counteracting growth inhibition by desoxypyridoxine varies from organism to organism. For Saccharomyces carlsbergensis the order of effectiveness was pyridoxine > pyridoxamine = pyridoxal. For Neurospora sitophila this order was pyridoxine ≧ pyridoxal > pyridoxamine.

Vitamin B6 antagonists and growth of microorganisms. II. 5-Desoxypyridoxal and related compounds.

Abstract Three 5-desoxy analogs of vitamin B6 were compared with 4-desoxypyridoxine and ω-methylpyridoxine as antagonists of vitamin B6. Except for minor variations, the order of decreasing effectiveness as antagonists of vitamin B6 for Saccharomyces carlsbergensis was: ω-methylpyridoxine > 4-desoxypyridoxine > 5-desoxypyridoxine ≧ 5-desoxypyridoxal > 5-desoxypyridoxamine. Pyridoxine was more effective than pyridoxal in preventing the inhibitory effects of these antagonists; pyridoxal in turn was more effective than pyridoxamine. For Streptococcus faecalis, 5-desoxypyridoxamine was an effective antagonist of vitamin B6; 5-desoxypyridoxal was somewhat less active; the remaining compounds were inactive. Pyridoxamine was more effective than either pyridoxal or pyridoxamine phosphate in preventing growth inhibition by these antagonists. The antagonists did not inhibit growth when vitamin B6 was made nonessential by the addition of d -alanine to the medium. For Lactobacillus helveticus, only 5-desoxypyridoxal proved effective as an antagonist of vitamin B6. The inhibitory effects on Saccharomyces carlsbergensis of 4-desoxypyridoxine and ω-methylpyridoxine were additive, indicating that these two compounds inhibited the same reaction in this organism. In contrast, the effects of 4-desoxypyridoxine (or of ω-methylpyridoxine) and of 5-desoxypyridoxal were exerted independently of one another; these inhibitors, therefore, appear to act by independent mechanisms.

The incorporation of 32P-labelled orthophosphate into nucleotides☆

Abstract 1. 2. Chemical fractionation, column and paper chromatographic methods have beee employed to study the short-term incorporation of 32P-orthophosphate into various nucleotides and other phosphorylated compounds in respiring rabbit liver homogenates. 2. 2. No evidence could be obtained for the labelling of flavin or pyridine nucleotides. AMP does not appear to be labelled, in contrast to ADP and ATP which are labelled. CoA, or one of its derivatives, is probably labelled. 3. 3. An unidentified compound, possibly an adenosine polyphosphate, is labelled.