The irreversible reaction between liver esterases and the active-site-directed inhibitor bis(4-nitropheny1)phosphate can be used in vivo both for the estimation of the esterase contents and for the measurement of the esterase degradation rates. A method based on this reaction is described which allows the simultaneous estimation of the rate constants of degradation and synthesis of esterases during a period of change in protein concentration. Rat liver was found to contain about 1 mg of organophosphate-binding esterases per g of fresh tissue while the microsomal fraction contains about 30 mg of esterases per g of microsomal protein. Esterase degradation and de novo synthesis were shown to remain in equilibrium for a period of at least five days following the injection of 10 mg bis(4-nitro-[14C]phenyl)phosphate per kg. The decrease of the relative amount of labeled esterases with time was found to follow first-order kinetics yielding an average esterase degrading constant of 0.0165 hwhich corresponds to a half-life of 42 h. These data were confirmed by an independent experiment using one of the standard procedures for the estimation of degradation rates: ['4C]leucine was incorporated and one of the esterases was subsequently isolated by immuno-precipitation. Using isoelectric focussing and dodecyl sulfate electrophoretic methods, the various esterase isoenzymes appeared to have very similar, if not identical turnover rates.This method for the estimation of the turnover characteristics was applied to evaluate hormone effects on liver esterases. The time course of the contents and the turnover of liver esterases was measured under the influence of glucagon treatment in diabetic rats and under the influence of high doses of insulin. The esterase content decreased faster than the average content of microsomal protein under the influence of glucagon. The reverse effect was observed with insulin-treated rats. Both insulin and glucagon apparently reduced the intracellular esterase turnover in rat liver. Kinetic analysis of the results revealed that insulin mainly lowered the esterase degradation rate, though the rate of esterase synthesis might also have been restricted. In the glucagon-treated rats the de novo synthesis of esterases was strongly reduced.Liver carboxylesterases that are dealt with in this paper, are a group of closely related isoenzymes [l -31 bound to the endoplasmic reticulum of hepatocytes. Two independent groups have reported on the intracellular turnover rate of rat liver esterases [4,5]. In both cases the applied methods were indirect so that the results could only give a rough idea of the order of magnitude of the turnover rates.Most of the methods applied so far for estimation of the rates of metabolism of intracellular proteins

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