In illuminated rod outer segment membranes, GTP and guanosine 5'-[fyl-imido]triphosphate (p[NH]ppG) have reciprocal effects on cGMP phosphodiesterase (PDEase; 3':5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17) activity and cGMP binding to noncatalytic-sites on that enzyme. Two micromolar p[NH]ppG increased PDEase activity more than 2-fold while inhibiting cGMP binding more than 40%. Reduction of noncatalytic cGMP binding, which followed addition of p[NH]ppG, was not a result of PDEase activation. Both effects of p[NH]ppG were completely dependent on the presence of bleached rhodopsin. A heat-stable factor has been found to inhibit PDEase activity and also to stimulate cGMP binding to noncatalytic cGMP binding sites. Addition of p[NH]ppG reversed the effects of this factor on both PDEase activity and cGMP binding. During purification of this material, the activity peaks for both PDEase inhibition and activation of noncatalytic cGMP binding comigrated on both Blue Sepharose CL-6B column chromatography and sucrose density gradients centrifugation, suggesting that the same factor could be responsible for both inhibition of PDEase activity and enhancement of noncatalytic cGMP binding. Limited tryptic proteolysis of PDEase, which markedly reduced cGMP binding to the noncatalytic sites, and experiments using highly purified cAMP (free of cGMP) as substrate for PDEase showed that the binding of cGMP to noncatalytic sites was not required for the heat-stable inhibitory factor to inhibit PDEase activity. We discuss possible relationships between the regulation of PDEase and the binding of cGMP to noncatalytic sites.

No outside funding supported this study. The authors report no potential conflicts of interest relevant to this research. Lai and Ting contributed to study concept and design and collected the data, along with the other authors. Data interpretation was performed by Lai, Koh, Obi, Ho, and Ting. The manuscript was written and revised by Lai, Koh, and Ho, with assistance from Ting and Obi.

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Mutual diffusion coefficients were measured at 298.15 K for aqueous mixtures of 0.5 normalmol‐dm−3 ZnCl2 with normalKCl at concentrations of 0.5, 1.25, 2.0, and 4.0 mol‐dm−3, using Rayleigh interferometry. These systems correspond to a fully charged zinc‐chlorine storage battery at various supporting electrolyte concentrations. The ZnCl2 main term diffusion coefficient is nearly independent of concentration, whereas the normalKCl main term varies by one‐third. The ZnCl2 cross term is small and positive. The normalKCl cross term varies from moderately large and positive to quite large and negative as the normalKCl concentration increases. The experimental values cannot be reliably approximated by Nernst‐Hartley type equations, even with speciation assumptions. Mutual diffusion data, together with transference number and conductivity data (when available), can be used to calculate ion transport in systems with combined electrical and diffusive transport.