GTP (gamma S) and GDP (beta S) as electron donors: New wine in old bottles

G proteins are membrane-bound regulatory proteins which modulate the activi ty of ion channels and other effector systems. The GTP and GDP analogs GTP (gamma S) and GDP (beta S) have been used to study the role of G proteins i n numerous physiologic systems. The prolonged effects of these analogs have been thought to be due to the fact that they are nonhydrolyzable. However, in this paper we show that the GTP (gamma S) and GDP (beta S) analogs are potent reducing agents at physiologic pH. This observation suggests that pr evious data obtained using these compounds may need to be reinterpreted.