| Research Abstract |
The human cell surface antigen CD38, which has an amino acid sequence homologous to Aplysia ADP-ribosyl cyclase, is a single-transmembrane type II glycoprotein. CD38 catalyzes not only the hydrolysis of NAD^+, but also the formation and hydrolysis of cyclic adenosine diphosphoribose (cADPR), which is a novel candidate that mediates Ca^<2+> release from intracellular Ca^<2+> stores. An increase in the cellular cADPR content was suggested in various types of cells by bioassays of a Ca^<2+>-releasing activity and/or the measurement of its amount on column chromatography. However, these assays are not sufficient for precise determination of the cellular cADPR content in terms of specificity and sensitivity. In the present study, we first developed a radioimmunoassay (RIA) for the measurement of the cellular cADPR content. The present RIA method, which exhibits reasonable specificity for and sensitivity to cADPR with prior treatment of the sample with enzymes, was applied to studies on the
… More
possible involvement of CD38 in the formation of cellular cADPR.1.Aplysia ADP-ribosyl cyclase was categorized as a lyase rather than hydrolase. 2.The CD38 NADase activity was, but the ADP-ribosyl cyclase activity was not, inhibited by dithiothreitol. These results indicated that enzyme reactions catalyzed by the two enzymes were different from each other, though both cleaved the N-glycoside bond of NAD^+ resulting in the liberation of nicotinamide. 3.However, Zn^<2+> directly interacted with CD38 to convert its catalytic properties from NADase to ADP-ribosyl cyclase, probably due to prevention of the access of water molecule to an intermediate of the enzyme-substrate complex. 4.A marked increase in cellular cADPR was accompanied by retinoic acid-induced differentiation of HL-60 cells. 5.Moreover, a high level of cellular cADPR was observed in other leukemic cell lines, in which CD38 mRNA was expressed. Thus, CD38, which was initially identified as an NADase, appeared to be responsible for the formation of cellular cADPR. Less
|
