Localization and turnover studies of membrane nicotinamide adenine dinucleotide glycohydrolase in rat liver.

Cell fractionation experiments indicate that NAD glycohydrolase (EC 3.2.2.6) is a constituent enzyme of both the microsomal (endoplasmic reticulum) and plasmalemma membrane of the rat hepatocyte. The enzyme was solubilized by steapsin digestion from both fractions and subsequently purified -2000 over the homogenate and -500 over the microsomal fraction by successive DEAE-cellulose, Sephadex G-100 chromatography, and polyacylamide gel electrophoresis. The purified enzyme is a heterogeneous mixture of proteins with two or three major components. The apparent half-life of the purified NAD glycohydrolase was determined to be -18 days by the use of a double labeling procedure with uniformly labeled 14C-L-leucine and 3H-(4,5)-L-leucine. The results add another protein to the list of those (NADPH-cytochrome c reductase, cytochrome b5) known to turn over asynchronously in the endoplasmic reticulum membrane. No gross differences were found between the half-life of microsomal and plasmalemmal NAD glycohydrolase.