Serine Hydroxymethyltransferase from Mung Bean (Vigna radiata) Is Not a Pyridoxal-5'-Phosphate- Dependent Enzyme

Sukanya, Narasimhan and Vijaya, M and Savithri, HS and Radhakrishnan , AN and Rao , Appaji N (1991) Serine Hydroxymethyltransferase from Mung Bean (Vigna radiata) Is Not a Pyridoxal-5'-Phosphate- Dependent Enzyme. In: Plant Physiology, 95 (2). pp. 351-357.

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Abstract

Serine hydroxymethyltransferase from mammalian and bacterial sources is a pyridoxal-5'-phosphate-containing enzyme, but the requirement of pyridoxal-5'-phosphate for the activity of the enzyme from plant sources is not clear. The specific activity of serine hydroxymethyltransferase isolated from mung bean (Vigna radiata) seedlings in the presence and absence of pyridoxal-5'-phosphate was comparable at every step of the purification procedure. The mung bean enzyme did not show the characteristic visible absorbance spectrum of pyridoxal-5'-phosphate protein. Unlike the enzymes from sheep, monkey, and human liver, which were converted to the apoenzyme upon treatment with L-cysteine and dialysis, the mung bean enzyme similarly treated was fully active. Additional evidence in support of the suggestion that pyridoxal-5'-phosphate may not be required for the mung bean enzyme was the observation that pencillamine, a well-known inhibitor of pyridoxal-5'-phosphate enzymes, did not perturb the enzyme spectrum or inhibit the activity of mung bean serine hydroxymethyltransferase. The sheep liver enzyme upon interaction with O-amino-D-serine gave a fluorescence spectrum with an emission maximum at 455 nm when excited at 360 nm. A 100-fold higher concentration of mung bean enzyme-O-amino-D-serine complex did not yield a fluorescence spectrum. The following observations suggest that pyridoxal-5'-phosphate normally present as a coenzyme in serine hydroxymethyltransferase was probably replaced in mung bean serine hydroxymethyltransferase by a covalently bound carbonyl group: (a) inhibiton by phenylhydrazine and hydroxylamine, which could not be reversed by dialysis and or addition of pyridoxal-5'-phosphate; (b) irreversible inactivation by sodium borohydride; (c) a spectrum characteristic of a phenylhydrazone upon interaction with phenylhydrazine; and (d) the covalent labeling of the enzyme with substrate/product serine and glycine upon reduction with sodium borohydride. These results indicate that in mung bean serine hydroxymethyltransferase, a covalently bound carbonyl group has probably replaced the pyridoxal-5'-phosphate that is present in the mammalian and bacterial enzymes.

Item Type:	Journal Article
Publication:	Plant Physiology
Publisher:	American Society of Plant Biologists
Additional Information:	Copyright of this article belongs to American Society of Plant Biologists.
Department/Centre:	Division of Biological Sciences > Biochemistry
Date Deposited:	30 Nov 2010 07:54
Last Modified:	30 Nov 2010 07:54
URI:	http://eprints.iisc.ac.in/id/eprint/34126

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