Two purified carboxylesterases that were isolated from a rat liver microsomal fraction in a Norwegian and a German laboratory were compared. The Norwegian enzyme preparation was classified as palmitoyl-CoA hydrolase (EC 3.1.2.2) in many earlier papers, whereas the German preparation was termed monoacylglycerol lipase (EC 3.1.1.23) or esterase pI 6.2/6.4 (non-specific carboxylesterase, EC 3.1.1.1). Antisera against the two purified enzyme preparations were cross-reactive. The two proteins co-migrate in sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Both enzymes exhibit identical inhibition characteristics with Mg2+, Ca2+ and bis-(4-nitrophenyl) phosphate if assayed with the two substrates palmitoyl-CoA and phenyl butyrate. It is concluded that the two esterase preparations are identical. However, immunoprecipitation and inhibition experiments confirm that this microsomal lipase differs from the palmitoyl-CoA hydrolases of rat liver cytosol and mitochondria.

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