| Abstract: | We have investigated effects of temperature on the catalytic and allosteric properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver. Vmax for cAMP and cGMP increased as assay temperature increased from 5 to 45 degrees C. At substrate concentrations below Kmapp, however, hydrolysis increased as temperature decreased from 45 to 5 degrees C and was much greater at 5 degrees C than at 45 degrees C. As assay temperature decreased, Kmapp for cAMP and cGMP decreased. Hill coefficients for cAMP and cGMP were approximately 1.9 at 45 degrees C and 1.2-1.0 at 5 degrees C. cGMP stimulated hydrolysis of 0.5 microM 3H]cAMP at all assay temperatures. Although maximal activity stimulated by cGMP, like Vmax, was lowest at 5 degrees C, presumably because of the effect of temperature on catalytic activity, the apparent activation constant (K alpha app) for cGMP stimulation was lower at 5 degrees C than at 45 degrees C. Thus, affinity for both substrate and effector was increased at 5 degrees C, suggesting that low temperature promotes transitions of the cGMP-stimulated phosphodiesterase to a "high affinity" state. That cGMP stimulated cAMP hydrolysis at 5 degrees C suggests that temperature-induced transitions are incomplete and/or readily reversible. In assays at 30 degrees C competitive inhibitors, like substrates, induce allosteric transitions which result in enhanced hydrolysis of low substrate (1.0 microM 3H] cAMP) concentrations. At higher substrate concentrations (50 microM 3H]cAMP), with the enzyme in the "activated" state, inhibitors compete with substrate at catalytic sites and reduce hydrolysis. At 45 degrees C, as at 30 degrees C, 1-methyl-3-isobutylxanthine (IBMX) and papaverine increased hydrolysis of 1.0 microM 3H]cAMP and reduced hydrolysis of 50 microM 3H]cAMP. At 5 degrees C, however, IBMX and papaverine inhibited hydrolysis of both 1.0 and 50 microM 3H]cAMP. Enzyme activity was relatively more sensitive to inhibition by IBMX at 5 degrees C than at 45 degrees C. Taken together, these observations support the notion that low temperature induces incomplete or readily reversible transitions to the high affinity state for substrates, effectors, and inhibitors. These observed effects of temperature also point out that enzyme determinants and topographical features responsible for transitions to the high affinity state and expression of catalytic activity can be regulated independently. |
