Carboxylesterases (CEs) are traditionally regarded as xenobiotic metabolizing enzymes that hydrolyze esterified xenobiotics to alcohol and carboxylic acid products. However, there is a growing appreciation for the role of CEs in the processing of endobiotics, including cholesteryl esters and triacylglycerols. Human liver microsomes (HLMs) are often used in reaction phenotyping studies to discern interindividual variability in xenobiotic metabolism. The two major CE isoforms expressed in human liver are hCE1 and hCE2. These two isoforms are different gene products. We have begun studies to evaluate the CE phenotype'' of human liver samples, i.e. to determine both the levels of hCE1 and hCE2 protein and the hydrolytic activity of each. We have previously shown that there is little variation in hCE1 protein expression in HLM samples from 11 individuals [a 1.3-fold difference between the highest and lowest individuals; coefficient of variation (CV), 9%]. hCE2 protein expression in individual HLMs is only slightly more variable than hCE1 (2.3-fold difference between the highest and lowest individuals; CV, 36%). However, hCE1 protein is found in 46-fold higher amounts in HLMs than hCE2 protein (64.4 +/- 16.5 microg hCE1/mg microsomal protein compared to 1.4 +/- 0.2 microg hCE2/mg microsomal protein). The hydrolytic activity specifically attributable to hCE1 and hCE2 in individual HLMs was measured using bioresmethrin (a pyrethroid insecticide hydrolyzed specifically by hCE1, but not by hCE2) and procaine (an analgesic drug hydrolyzed by hCE2, but not by hCE1). The hydrolytic activity of individual HLMs toward bioresmethrin and procaine did not correlate with the protein content of hCE1 and hCE2. Thus, the mere abundance of CE proteins is not a good predictor of CE activity in HLMs. Identification of the factors that lead to altered CE activities in HLMs will be important to characterize since several pharmaceutical agents, environmental toxicants, and endobiotics are metabolized by these enzymes.