Conroy C W, Maren T H
University of Florida College of Medicine, Department of Pharmacology and Therapeutics, Gainesville 32610, USA.
Mol Pharmacol. 1995 Sep;48(3):486-91.
We report the effect of temperature on the equilibrium dissociation constants (Kl) for a series of six sulfonamides binding to three carbonic anhydrase (CA) isoenzymes (I, II, and IV). Kl values obtained at 0 degree, 15 degrees, and 23 degrees under conditions of nearly constant and low substrate (CO2) concentration were used to calculate enthalpy and entropy changes associated with sulfonamide binding as well as to provide estimates of inhibitory potency of sulfonamides at 37 degrees. We studies four classic sulfonamide (methazolamide, benzolamide, ethoxzolamide, and sulfanilamide) and the novel sulfonamides MK-507 (dorzolamide) and CF3SO2NH2. In all cases, the Kl was observed to increase with increasing temperature, which is consistent with a negative enthalpy of sulfonamide binding. The extrapolated increase in Kl over the 0-37 degrees temperature range varied from 4-fold for sulfanilamide binding to CA l to 14-fold for CF3SO2NH2 binding to CA IV, corresponding to binding enthalpy values of -7.2 to -11.7 kcal/mol. For CA II and I, entropy changes associated with sulfonamide binding were in general modest and ranged from -5.3 to +4.1 entropy units (eu) for five of the compounds tested. In contrast, ethoxzolamide binding was associated with a relatively large positive entropy change. Also, the variatione in k(on) and k(off) with temperature were studied for three sulfonamides binding to CA II. The association rate constants for methazolamide, benzolamide, and ethoxzolamide binding showed increases of 2-fold or less, whereas dissociation constants increased 3-9-fold over the range of 0-37 degrees. Thus, the temperature effect in increasing Kl is in large part due to a faster rate of sulfonamide dissociation. Apparent activation parameters at 23 degrees for k(on) were delta H++ = -2.35 to 3.8 kcal/mol, delta G++ = 7.3 to 8.6 kcal/mol, and delta S++ = -16.2 to -32.7 entropy units. For k(off), the corresponding values were delta H++ = 5.6 to 14.5 kcal/mol, delta G++ = 19.0 kcal/mol, and delta S++ = -14.8 to -45.7 entropy units.
我们报告了温度对一系列六种磺胺类药物与三种碳酸酐酶（CA）同工酶（I、II和IV）结合的平衡解离常数（Kl）的影响。在底物（CO2）浓度近乎恒定且较低的条件下，于0℃、15℃和23℃获得的Kl值用于计算与磺胺类药物结合相关的焓变和熵变，以及估算磺胺类药物在37℃时的抑制效力。我们研究了四种经典磺胺类药物（甲醋唑胺、苯甲酰胺、乙氧唑胺和磺胺）以及新型磺胺类药物MK - 507（多佐胺）和CF3SO2NH2。在所有情况下，均观察到Kl随温度升高而增加，这与磺胺类药物结合的负焓变一致。在0 - 37℃温度范围内，Kl的外推增加倍数从磺胺与CA I结合的4倍到CF3SO2NH2与CA IV结合的14倍不等，对应结合焓值为 - 7.2至 - 11.7千卡/摩尔。对于CA II和I，测试的五种化合物中，与磺胺类药物结合相关的熵变通常较小，范围为 - 5.3至 + 4.1熵单位（eu）。相比之下，乙氧唑胺的结合与相对较大的正熵变相关。此外，还研究了三种磺胺类药物与CA II结合时k（on）和k（off）随温度的变化。甲醋唑胺、苯甲酰胺和乙氧唑胺结合的缔合速率常数增加2倍或更少，而解离常数在0 - 37℃范围内增加3 - 9倍。因此，Kl增加的温度效应在很大程度上是由于磺胺类药物解离速率加快。23℃时k（on）的表观活化参数为ΔH++ = - 2.35至3.8千卡/摩尔，ΔG++ = 7.3至8.6千卡/摩尔，ΔS++ = - 16.2至 - 32.7熵单位。对于k（off），相应的值为ΔH++ = 5.6至14.5千卡/摩尔，ΔG++ = 19.0千卡/摩尔，ΔS++ = - 14.8至 - 45.7熵单位。
