In recent years, evidence has accumulated that NAD(+) serves as a precursor
of metabolites that are involved in a number of regulatory processes. In t
his work we show that extracellularly added NAD(+) was rapidly degraded by
intact human monocytes to nicotinamide and ADP-ribose. Besides these main p
roducts, minor amounts of AMP, ADP and cADP-ribose were formed. Expression
of CD38, which has been identified as NAD(+)-glycohydrolase (EC 3.2.2.6) de
grading NAD(+) into nicotinamide and ADP-ribose, was determined on freshly
isolated human monocytes by flow cytometry and RT-PCR. Upon ligation with a
nti-CD38 mAb, CD38 underwent internalization, shedding and new expression.
As monocytes possess an intracellular CD38 pool, it could serve as a source
for newly expressed CD38. Differentiation of monocytes to macrophages resu
lted in down-regulation of surface expression of CD38. This decrease correl
ates with a reduction in NADase activity, indicating that the amount of fun
ctional active CD38 molecules decrease during differentiation. As CD38 mRNA
was found to be diminished in macrophages, regulation of the gene product
seems to occur at the level of transcription or mRNA stability.