Human Erythrocyte Monoester Lipase: Characterization and Radiochemical Assay of the Cell-Bound Enzyme in Normal Subjects*

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Abstract
A monoester lipase (MEL) activity (EC 3.1.1.3) is described in human red blood cells (RBC). The lipase acts as a cell-bound enzyme and is able to exert its catalytic activity in vitro toward an exogenously added emulsified substrate. The enzyme activity, which appears to be confined to the cell membrane, is inhibited by Triton X-100. The MEL activity of human RBC is assayed using intact RBC as the enzyme source, with an emulsion of ethyl [3]oleate (2 mM) as the substrate. The optimum pH for the reaction is 7.8 at 37 C. Lipolytic rates are monitored by quantitation of the mount of [3H]oleic acid released during 20 min of incubation after extraction by means of a liquid-liquid partition system. Suspensions of purified RBC obtained from 161 healthy adult subjects had a MEL activity of 1022 ± 134 mU/lO12 RBC (mean ± SD), with a normal range (±2 SD) between 754–1290 mU. The individual activity values varied from 733–1490 mU. The median of the 161 subjects was 1010 mU/1012 RBC. There was no significant difference between the mean activities of RBC samples from men and women. MEL activity in RBC from the cord blood of 16 normal infants was found to be 43% higher than that in adults, with an average activity of 1458 ± 174 mU (mean ± SD).

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