NITRIC-OXIDE AND NAD-DEPENDENT PROTEIN MODIFICATION

Nitric oxide (NO) has been suggested to act as a regulator of endogeno us intracellular ADP-ribosylation, based on radiolabelling of proteins in tissue homogenates incubated with [P-32]NAD and NO. After the NO-s timulated modification was replicated in a defined system containing o nly the purified acceptor protein, glyceraldehyde-3-phosphate dehydrog enase (GAPDH), the hypothesis of NO-stimulation of an endogenous ADP-r ibosyltransferase became moot. The NO-stimulated, NAD-dependent modifi cation of GAPDH was recently characterized as covalent binding of the whole NAD molecule to the enzyme, not ADP-ribosylation. With this resu lt, along with the knowledge that GAPDH is stoichiometrically S-nitros ylated, the role of NO in protein modification with NAD may be viewed as the conferring of an unexpected chemical reactivity upon GAPDH, pos sibly due to nitrosylation of a cysteine in the enzyme active site.