Journal
NEURON
Volume 27, Issue 3, Pages 525-537Publisher
CELL PRESS
DOI: 10.1016/S0896-6273(00)00063-5
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Funding
- NEI NIH HHS [EY-00463, EY-10336, EY-02660] Funding Source: Medline
- NATIONAL EYE INSTITUTE [R01EY010336, R01EY002660] Funding Source: NIH RePORTER
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We have resolved a central and long-standing paradox in understanding the amplification 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 molecules and phosphodiesterase (PDE) catalytic subunits at rates of 120-150/s, much lower than indirect estimates from lightscattering experiments. Further, we measure the Michaelis constant, K-m, of the rod PDE activated by transducin to be 10 mu M, at least 10-fold lower than published estimates. Thus, the gain of cGMP hydrolysis (determined by k(cat)/K-m) is at least 10-fold higher 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.
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