| Abstrakt: |
The Pi-ATP exchange and ATP hydrolytic reactions, by the F0F1complex, were studied in Rhodospirillum rubrumchromatophores in the dark. An optimal pH between 7.0 and 8.5 was determined for the hydrolytic and exchange reactions. Under these conditions, the hydrolysis/exchange ratio was approximately 2. The kinetic analysis of the hydrolytic and exchange reactions using Mg-ATP as substrate showed a change in the hydrolysis/exchange ratio that varied between 2.0 and 2.8 as the substrate concentration was increased. With Ca-ATP, hydrolysis was not saturated up to a substrate concentration of 5.0 mM, and the hydrolysis/exchange ratios changed from 2 to 240 as the substrate concentration was increased from 0.06 to 5.0 mM. Free Mg2+inhibited hydrolysis and phosphate uptake without altering the hydrolysis/exchange ratio. Nigericin induced an increase in the hydrolysis/exchange ratio from 2.7 to 130, whereas in the presence of valinomycin, this ratio increased from 2.7 to 21. From these results, it can be concluded that Ca-ATP hydrolysis is loosely coupled to phosphate uptake given that Pi-ATP exchange activity is extremely low, even at high rates of ATP hydrolysis. |
