We have investigated the following enzyme systemsin extracts of a wild type strain of Neurospora:
1) Threonine deaminase. It catalyzes the reactiongiving rise to alpha-ketobutyric acid and ammonia fromthreonine. It was concluded that alpha-ketobutyric acid,glutamic acid or a deaminated alpha-ketobutyric acid precursorin equilibrium with alpha-ketobutyric could not beintermediates in this reaction. Pyridoxal phosphate activates the system, and a number of methods were tested inorder to improve the resolution of enzyme and coenzyme inthe preparations.
2) Serine deaminase. Yields pyruvic acid and ammoniafrom serine, and is also activated by pyridoxal phosphate.The responses of serine and threonine deaminases topyridoxal phosphate are at variance, suggesting that twodifferent enzymes are involved.
The effect of pH and temperature on serine and threonine deaminase was investigated.
3) Glutamic-alphaketobutyric transaminase, which is activated by pyridoxal phosphate.
4) A system forming alpha-aminobutyric from threonine, possibly as a result of the summation of activities 1) and 3).
5) A system forming an unidentified blue fluorescent product by incubation with threonine, but not with any ofa number of related metabolites (serine included).
6) Alpha-ketobu tyric decarboxylase, which is activated by cocarboxylase, and has a pH optimun1 of 5.5.
The threonine deaminase activities of a number of threonineless strains and of a B6-less strain were compared with those of wild type, using cultures grown underdifferent conditions. The significance of the variability in activity encountered is discussed. Mutant 35423, which requires threonine for growth but is unable to use alpha ketobutyric or alpha-aminobutyric acid,has the ability of converting threonine into those acids in vitro.
Mutant 44104 cannot utilize alpha-ketobutyric acid in place of alpha-aminobutyric to initiate early growth, but its glutamic-alpbaketobutyric transamtnase is as active in vitro as that of wild type.
A new scheme of threonine biosynthesis is presentedto account for the information available.
An attempt is made to find a common denominator to the mechanisms of the diverse coenzymatic activities of pyridoxal phosphate, and schemes for those mechanisms are proposed.
