ADP-ribosylation factors: protein activators of cholera toxin

Alterations of adenylyl cyclase-linked G proteins in rat liver during aging

beta-Adrenergic stimulation of adenylyl cyclase in rat liver increases during aging. We examined whether this increase is related to alterations in the stimulatory and inhibitory G proteins (Gs and Gi) linked to adenylyl cyclase. Levels of immunoreactive alpha- and beta-subunits of Ga and Gi in liver plasma membranes from 6-, 12-, 18-, and 24-mo-old rats were unchanged with age, as was pertussis toxin-catalyzed [32P]ADP ribosylation of Gi alpha. Cholera toxin-catalyzed [32P]ADP ribosylation of Ga alpha and Gs bioactivity, assessed as reconstitution of adenylyl cyclase activity in S49 cyc- cell membranes, increased two- to threefold between 6 and 12-18 mo, and declined by 24 mo. Recombinant ADP ribosylation factor (ARF) enhanced cholera toxin labeling of Gs alpha at all ages, yet abolished the increase in toxin labeling at 12-18 mo. Auto-ADP ribosylation of the cholera toxin A1 peptide also increased transiently with age. Alteration of Gs alpha, as reflected by increased cholera toxin labeling and Gs bioactivity, may be involved in the regulation of beta-adrenergic-responsive adenylyl cyclase in rat liver during aging. Moreover, changes in endogenous ARF levels could contribute to age differences in cholera toxin labeling of Gs alpha.