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.