The gain of rod phototransduction: Reconciliation of biochemical and electrophysiological measurements

We have resolved a central and long-standing paradox in understanding the a mplification of rod phototransduction by making direct measurements of the gains of the underlying enzymatic amplifiers. We find that under optimized conditions a single photoisomerized rhodopsin activates transducin molecule s and phosphodiesterase (PDE) catalytic subunits at rates of 120-150/s, muc h lower than indirect estimates from lightscattering experiments. Further, we measure the Michaelis constant, K-m, of the rod PDE activated by transdu cin to be 10 mu M, at least 10-fold lower than published estimates. Thus, t he gain of cGMP hydrolysis (determined by k(cat)/K-m) is at least 10-fold h igher than reported in the literature. Accordingly, our results now provide a quantitative account of the overall gain of the rod cascade in terms of directly measured factors.