| Popis: |
Although much attention has been paid to the group of closely related compounds making up 'vitamin B6' as a consequence of the great importance of this vitamin in amino acid metabolism, nothing is known of its biogenesis by competent organisms. That D-alanine might be incorporated into the pyridine ring of pyridoxine in Streptococcus faecalis, was disproved by the discovery that in fact em > D-alanine merely showed a vitamin B6-sparing action in an otherwise complete replacement medium. Although the suggestion has seen made that in a certain strain of yeast, thiamine and pyridoxine were interconvertible via a common pyrimidine precursory the supporting evidence was not convincing. A survey of the known routes of synthesis of N-heterocyclic compounds suggests that incorporation of the N atom is achieved by spontaneous condensation of ketone with amine groups so that the immediate aliphatic precursor of vitamin B6 might be expected to carry these two reactive groupings. At present, the only known route of pyridine ring biogenesis has as starting material the already aromatic benzene ring, i.e., origin of nicotinic, quinolinic and picolinic acids from 3-hydroxyanthranilic acid. There is no evidence, however, to suggest a common origin of nicotinic acid and vitamin B6, though it is true that in some microorganisms the pathway of nicotinic acid formation is still unknown, appraisal of the possible methods available for the use of microorganisms in a study of growth factor biosynthesis, suggests that the first objective should be the isolation of an organism capable of forming large amounts of vitamin B6. Failing this, attention should be directed to an examination of selected vitamin B6 auxotrophs. A modification of the reported assay of vitamin B6 using Sacoharomyces carlsbergensis 4238 allowed smaller volumes to be used in static culture. As a direct consequence of its being a 'rate assay' (where rates of growth rather than its final possible extent is proportional to the concentration of growth factor), it was essential to maintain a standard time and steady temperature of incubation, and a constant degree of aeration and inoculum size, though it had the advantage that the sensitivity of the method could (within limits) be varied by adjusting the time of incubation. With the strain of S. carlsbergensis 4228 used, thiamine 'spared' vitamin B6 at low concentrations of the latter, and was added in excess to the culture medium. Using this method of assay, it was discovered that all of a number of various bacteria grown in chemically-defined media synthesised uniformly small amounts of vitamin B6. The best synthesis obtained was with a strain of Proteus morganii whose production of 4μmole vitamin B6/g. wt. of organism was only about ten times greater than average. |
