NAD glycohydrolases are the longest known enzymes that catalyze ADP-ri
bose transfer. The function of these ubiquitous, membrane-bound enzyme
s has been a long standing puzzle. The NAD glycohydrolases are briefly
reviewed in light of the discovery by our laboratory that NAD glycohy
drolases are bifunctional enzymes that can catalyze both the synthesis
and hydrolysis of cyclic ADP-ribose, a putative second messenger of c
alcium homeostasis.