期刊名称：Journal of Nutritional Science and Vitaminology
印刷版ISSN：0301-4800
电子版ISSN：1881-7742
出版年度：1973
卷号：19
期号：4
页码：321-332
DOI：10.3177/jnsv.19.321
出版社：Center for Academic Publications Japan
摘要：The coenzymatic activities of 2-nor-2-hydroxymethyl-pyridoxal 5'-phosphate (2'-hydroxy PLP) for various vitamin B6-dependent enzymes were studied in order to obtain information on the role of the methyl group at position 2 of pyridoxal 5'-phosphate (PLP) for the binding to apoprotein and for the catalytic action of the holoenzyme. Although it has been demonstrated that this coenzyme analog binds to the catalytic center and serves as a coenzyme for a variety of B6 en-zymes, the mode of binding as well as the catalytic activity of the holo-enzyme reconstituted with the analog was different according to the kinds of enzyme and assay conditions. In the ordinary assay system using Tris-HCl buffer, 2'-hydroxy PLP showed almost an equal affinity for pig heart muscle mitochondrial and cytoplasmic aspartate aminotransferase (GOTs, and GOTm) to that of PLP. The maximal velocity ( V max value) of the reaction mediated by GOTs, or GOTm reconstituted with 2'-hydroxy PLP was somewhat large as compared with that catalyzed by the native holoenzyme. How-ever, when phosphate buffer was used for the assay system, the affinity of the coenzyme analog decreased markedly and the V max value catalyzed by the analog-bound GOTs, or GOTm became approximately 70% of that by the native enzyme. Thus as judged from the V max value under ordinary assay conditions, 2'-hydroxy PLP is considered to be rather superior to PLP as the coenzyme for GOT. Phosphate anion inhibited the binding of PLP or 2'-hydroxy PLP to apoGOT in a competitive manner and further caused an appreciable decrease in the apparent V max of the reaction catalyzed by the resulting holoenzyme. The K 1, values of phosphate anion against PLP and 2'-hydroxy PLP were 11.9 and 3.75, respectively. On the other hand, in the case of E. coli tryptophanase, both the affinity for 2'-hydroxy PLP and the V max value of the reaction mediated by the analog-bound enzyme were significantly smaller than those of the native coenzyme irrespective of the buffer used. Of the other B6 enzymes tested hitherto, E. intermedia tyrosine phenol-lyase and Pseudomonas dacunhae aspartate β-decarboxylase showed the same behaviors toward 2'-hydroxy PLP as that of tryptophanase. On the contrary, the coenzyme analog served as a more efficient co-enzyme than the native coenzyme for E. coli D-serine dehydrase in the presence of phosphate anion. These results indicate that there are at least three groups of B6 enzymes exhibiting different interaction with the 2-methyl group of PLP.
