AbstractThe cell surface antigen, CD38, is a bifunctional ecto‐enzyme, which is predominantly expressed on hematopoietic cells during differentiation. In the present study, it is shown that hemin treatment of K562 cells gives rise to induction of enzymatic activities inherent to CD38. GDP‐ribosyl cyclase activity, an indicator of CD38, increased initially in response to hemin in a time‐dependent manner, reached a maximum level on the 5th day and, thereafter, declined sharply to the initial level. The increase in NAD+ glycohydrolase and ADP‐ribose uptake activities followed a similar time course. However, the decline in the latter activities after the 5th day of induction appeared to be rather slow in contrast to GDP‐ribosyl cyclase activity. The time course of these changes was well correlated with the FACScan findings obtained by use of anti‐CD38 monoclonal antibody. SDS–PAGE and Western blot analyses by use of the monoclonal antibody OKT10 revealed a transient hemin‐dependent appearence of a 43 kDa membrane protein with maximum signal intensity on the first 4 days of incubation. There was subsequently a gradual decrease on the 5th day, concomitant with a reciprocal increase in activity of the internalized protein fraction. The results together indicated that hemin‐induced expression of CD38 was followed by its down‐regulation. J. Cell. Biochem. 90: 379–386, 2003. © 2003 Wiley‐Liss, Inc.