SUMMARY: The strain of used (strain 30) possessed decarboxylases specific for -histidine, -lysine and -ornithine. The rate of decarboxylation increased after a lag period to a maximum value which was dependent on the concentration of pyridoxal. A number of factors was found to affect the rate of activation of the apoenzymes, or, it is thought, probably the rate of pyridoxal phosphate synthesis. Among these factors were: the presence in the test system of the specific substrate for the decarboxylase, the concentration of pyridoxal, and the pH value of the test system. Cells grown in medium deficient in both pyridoxin and folic acid had greatly decreased activation rates for ornithine apodecarboxylase, and pyridoxal phosphate synthesis was also decreased. The addition of folic acid to the system did not affect the rate of activation with folic acid-deficient cells. The addition of thymine to the test system increased the activation rate for folic acid-rich cells, but had no effect with folic acid-deficient cells; other purines were ineffective. It is concluded that some product of thymine metabolism which folic acid-deficient cells are unable to make is concerned with the biological phosphorylation of pyridoxal by this organism.


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