Specificity of L(-)-tyrosine decarboxylase

[摘要] The affinity of the bacterial enzyme L(-)-tyrosine decarboxylase for a wide variety of substrates has once been implied by the result of this study. Like the mammalian aromatic L-amino acid decarboxylases, L(-)-tyrosine decarboxylase from Streptococcus faecalis was found to be dependent on pyridoxal-5;;-phosphate (PALP) as coenzyme for its activity. This was shown by doubling up of the rate of carbon dioxide (CO2) production when an incubation of the enzyme in the absence of PALP was compared with one in the presence of the coenzyme. The method found best for the extraction of enzyme from the acetonized intact cells was grinding with alumina. With a few modifications of the original procedure by Epps (26), a procedure for a 170-fold purification was developed. The pH optimum found for the enzyme was 5.5 and the variation of the enzyme activity with the enzyme concentration was found to be directly proportional. Two out of the 21 different analogs of the amino acids phenylalanine, tyrosine, and tryptophan were found to show affinity for L(-)-tyrosine decarboxylase, namely: L-3-aminotyrosine (3-ATy) and L-3-monoiodotyrosine (3-ITy). The rate of CO2 evolution using 3-ATy as substrate was about 30% that of the reference substrate, L(-)-tyrosine, and that of 3-ITy was approximately half as much as 3-ATy. This implies then that these two analogs are possible substrates for the enzyme. The difference in the rates of decarboxylation between 3-ATy and and 3-ITy suggests that the active group of the enzyme fits more the space in between p-hydroxy and m-amino substituted tyrosine than that between p-hydroxy and miodine substituted derivative. This seemed to follow the same lining that was suggested by Blaschlco (l1) wherein the active site of the bacterial decarboxylase was postulated to fit in the space between the p- and m-substituents of the substrate whereas that of mammalian enzyme fits that space between the m- and o-substituents. This then led to his conclusion that while bacterial enzyme decarboxylates both p- and m-substituted aromatic amino acids, mammalian decarboxylase does the m- and o-substituted ones which suggests the similarities but not the exactness ci the two enzyme systems on account of the affinities of both for the m-substituted derivatives. The fact that both analogs are of the L-configuration and demonstrated affinity for the enzyme is an evidence for the stereospecificity of the faecal streptococcal enzyme, L(-)-tyrosine decarboxylase.

[发布机构] Rice University