Vanadate-Dependent Oxidation of Pyridine Nucleotides in Rat Liver Microsomal Membranes

Document Type

Article

Journal/Book Title/Conference

Archives of Biochemistry and Biophysics

Volume

255

Issue

2

Publisher

Elsevier

Publication Date

1987

First Page

267

Last Page

273

Abstract

An enzymatic Na3VO4-dependent system for the oxidation of reduced pyridine nucleotides in purified rat liver microsomes was characterized. The system has a pH optimum of 6.5, and appears to be specific for vanadate, since activity in the presence of a related transition metal, molybdate, was not detected. Vanadate-dependent oxidation occurred with a concomitant consumption of O2 and, contrary to previous reports, preferred NADPH over NADH. At pH 6.5, the NADPH/NADH oxidase activity ratio was greater than 2:1. Sodium vanadate-dependent oxidation of NADH was inhibited by rotenone, antimycin A, NaN3, and NaCN. Conversely, Na3VO4-dependent NADPH oxidation was slightly affected by rotenone, but was insensitive to antimycin A, NaN3, NaCN, or quinacrine. Vanadate-dependent oxidation of either pyridine nucleotide was inhibited by the addition of either Superoxide dismutase or catalase, indicating that both superoxide and hydrogen peroxide may be intermediates in the process. Linear sucrose gradient purification of the microsomes showed that the vanadate-dependent system for NADPH oxidation resides primarily in the endoplasmic reticulum. These studies indicate the existence of separate and distinct enzymatic systems for vanadate-stimulated oxidation of NADPH and NADH in mammalian microsomal membranes, and argue against an exclusive role of endogenous Superoxide in the process.

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