| Abstract: | We previously demonstrated that the hydrolysis of GTP by canine cardiac sarcoplasmic reticulum is not sensitive to calcium and does not support the translocation of calcium and oxalate into the vesicular space. In response to GTP, however, calcium is accumulated into a compartment which is sensitive to pH and ionophore. In the present paper, we further explored the relationship between GTP hydrolysis and GTP-induced calcium accumulation. Both ATP- and GTP-induced calcium accumulation were prevented by the sulfhydryl reagent, N-ethylmaleimide (NEM; I50 = 0.2 mM). In contrast, the sensitivity of NTP hydrolysis to NEM differed markedly; GTPase activity was not affected by NEM, whereas ATPase activity was markedly inhibited. Conversely, although the GTPase was noncompetitively inhibited by the ATP analogue, adenylyl imidodiphosphate (Ki = 8 microM), and was competitively inhibited by the GTP analogue, guanylyl imidodiphosphate (Ki = 60 microM), GTP-induced calcium accumulation was not affected by the NTP analogues at any concentration. Therefore, the GTP-dependent accumulation of calcium into the pH- and ionophore-sensitive compartment of cardiac SR may not require GTP hydrolysis but may be dependent on GTP binding. The previously reported noncompetitive inhibition of the GTPase by ATP was also observed when the calcium-dependent hydrolysis of ATP was prevented by NEM (Ki = 1.2 microM). Along with the noncompetitive inhibition of the GTPase by adenylyl imidodiphosphate, the inhibition of the GTP by ATP in the presence of NEM suggests that ATP binding may be involved in the observed inhibition. The Ki for the noncompetitive inhibition of GTPase activity is compatible with ATP binding to the high affinity catalytic site of the ATPase. Thus, although GTP-induced calcium accumulation differs somewhat from ATP-dependent calcium translocation, the similarities between the two processes (i.e. similar time courses and sensitivity to pH, ionophore, and sulfhydryl modification) suggest that they may be related in some manner. |
