| Abstract: | The distribution of nicotinamide adenine dinucleotide (NAD) glycohydrolase in rat liver was investigated by subcellular fractionation and by isolation of hepatocytes and sinusoidal cells. The behavior of NAD glycohydrolase in subcellular fractionation was peculiar because, although the enzyme was mainly microsomal, plasma membrane preparations contained distinctly more NAD glycohydrolase than could be accounted for by their content in elements derived from the endoplasmic reticulum or the Golgi complex identified by glucose-6-phosphatase and galactosyltransferase, respectively. When microsomal and plasmalemmal preparations were brought to equilibrium in a linear-density gradient, NAD glycohydrolase differed from these enzymes and behaved like 5'-nucleotidase and alkaline phosphodiesterase I. NAD glycohydrolase was markedly displaced towards higher densities after treatment with digitonin. This behavior in density-gradient centrifugation strongly suggests that NAD glycohydrolase is an exclusive enzyme of the plasma membrane. NAD glycohydrolase differed clearly from other plasmalemmal enzymes when the liver was fractionated into hepatocytes and sinusoidal cells; its specific activity was considerably greater in sinusoidal cell than in hepatocyte preparations. Further subfractionation of sinusoidal cell preparations into endothelial and Kupffer cells by counterflow elutriation showed that NAD glycohydrolase is more active in Kupffer cells. We estimate that the specific activity of NAD glycohydrolase activity is at least 65-fold higher at the periphery of Kupffer cells than at the periphery of hepatocytes. As the enzyme shows not structure-linked latency and is an exclusive constituent of the plasma membranes, we conclude that it is an ectoenzyme that cannot lead to a rapid turnover of the cytosolic pyridine nucleotides. |
