The rate of GTP hydrolysis on p21ras is accelerated by approximately 10(5) times by the catalytic domains of GTPase-activating proteins (GAPs), p120-GAP (GAP-344) or neurofibromin (NF1-334). The kinetic mechanism of this activation has been investigated by following the release of inorganic phosphate (Pi), using a fluorescent probe that is sensitive to Pi [Brune, M., Hunter, J., Corrie, J. E. T., & Webb, M. R. (1994) Biochemistry 33, 8262-8271]. Measurements were made in real time with a stopped-flow apparatus, in which the p21ras complex with the 2',3'-methanthraniloyl analogue of GTP (mantGTP) was mixed with the GAP in the presence of this Pi probe. The results show that Pi release is fast and that the overall hydrolysis is controlled by the cleavage itself or a conformational change preceding the cleavage. The time courses were single exponentials over a range of [GAP-344] and were modeled to show that a single step controlled Pi release. The maximum rate constant was 15 s-1 (all data at 30 degrees C, pH 7.6, low ionic strength) in experiments in which GAP-344 underwent a single turnover, compared with 5 s-1 for multiple-turnover experiments, and possible causes of this discrepancy were investigated and discussed. With NF1-334 the time courses were more complex, showing a lag prior to rapid release of Pi. The results were consistent with a Kd of 0.04 microM for NF1-344 affinity is some 3 orders of magnitude tighter than that of GAP-344.(ABSTRACT TRUNCATED AT 250 WORDS)