A positional isotope exchange study of the argininosuccinate lyase reaction.

15N nuclear magnetic resonance spectroscopy was used to follow the positional isotope exchange reaction of bovine liver argininosuccinate lyase. The enzyme was shown to catalyze the N-3-N-2 positional nitrogen exchange in [3-15N]argininosuccinate in the presence of excess arginase. The ratio of the positional isotope exchange rate and the rate for net substrate turnover is less than 0.15 at low levels of fumarate but increases to a limiting value of 1.8 at high fumarate. These data have been interpreted to mean that the dissociation of fumarate and arginine from the ternary enzyme complex is random although fumarate is released at least an order of magnitude faster than is arginine from this complex. The rate constant for the release of fumarate from enzyme-arginine-fumarate is at least 6 times faster than the turnover number of the reverse reaction of argininosuccinate lyase. The lower limit for the release of arginine from this same complex is 0.5.