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Abstract

The effect of temperature on sulfate equilibrium exchange was measured in cells which were modified by papain, pronase, salicylate, or i-fluoro-2,4-dinitrobenzene (FDNB).

Following modification at 37°C with the enzymes of FDNB the excess modifiers were removed and sulfate exchange was measured at 4°C and 34°C. The reversibly bound salicylate was also present during the exchange.

With increasing concentrations of salicylate of papain, SO₄²⁻-permeability shows a saturation type decrease at both temperatures. The apparent activation enthalpy (E_A) does not change significantly.

After treatment with FDNB or pronase, saturation type inhibition is observed at 37°C. If measured at 4 °C, sufficiently high concentrations of both modifiers accelerate SOSO₄²⁻ transfer. At pH 7.2, E_A decreases from 33 to 10 kcal per mole per °K.

If measured at 37 °C, in FDNB or pronase treated cells the rate of anion movement decreases with increasing pH. If measured at 4°C, the pH dependence is reversed. E_A is correspondingly lowered with increasing pH down to 6.4 kcal per mole per °K at pH 8.3. In contrast, the pH dependence of untreated cells is generally similar at both temperatures. At 37 °C a maximum is observed at pH 6.2. This is shifted to pH 5.8 at 4 °C. Variations of E_A with pH are relatively small over the range pH 5.8 to 8.0.

A study of the effects of purified pronase subfractions showed that neither aminopeptidase nor carboxypeptidase activity is responsible for the change of E_A. Among the endopeptidases those with activity towards casein and the ester p-nitrophenyl acetate were found to be particularly potent.