Protein kinase and NO-stimulated ADP-ribosyltransferase activities associated with glyceraldehyde-3-phosphate dehydrogenase isolated from human liver.
Background/aims: GAPD has been exhaustively investigated as a key cytosolic enzyme in glycolysis. In recent years GAPD has also been implicated in many cellular activities unrelated to glycolysis. However, although various functions have been ascribed to GAPD from rabbit muscle, human blood and rat tissues, no information is available on human liver GAPD. We have recently demonstrated that, as a cellular kinase, GAPD might interfere in the life-cycle of hepatitis B virus. We therefore investigated the enzymatic activities and subcellular localization of GAPD in normal human liver. Methods: GAPD and hepatocyte membranes were isolated from human liver homogenates to study the subcellular localization and enzymatic activities of GADP (kinase and ADP-ribosyltransferase). Results: (i) GAPD was recovered from the plasma-membrane-enriched fraction, in internal membranes, and in the cytosol; (ii) GAPD could be phosphorylated, a phenomenon inhibited by both GAP and NADH; and (iii) GAPD exhibits ADP-ribosyltransferase activity, which is stimulated by nitric oxide in a concentration-dependent manner. Conclusions: Human liver GAPD may play significant biological roles in addition to glycolysis, especially in signal transduction and in intracellular processes possibly involved in HBV infection.