Oil-induced mononuclear phagocytes (MN) were quantitatively assayed for various hydrolases as unfractionated suspensions of frozen and thawed cells. They apparently contain two proteases. The first, measured with urea- or acid-denatured hemoglobin, was similar to purified Proteinase I of lung with respect to pH optimum (pH 4), stability, hydrolytic and polymerizing activities, and reactions to various inhibitors. The second protease resembled chymotrypsin in its hydrolysis of glycyl-L-phenylalanine amide, acetyl-L-tyrosine ethyl ester and N-benzoyl-DL-phenylalanine-ß-naphthol ester (BPN). With the latter, its pH optimum was between 5.0 and 5.8, and its action was inhibited by diisopropylphosphorofluoridate (DFP) and p-chloromercuribenzoate. When assayed under the above conditions, polymorphonuclear exudate cells (PMN) and red blood corpuscles (RBC) manifested little or no hydrolysis of either hemoglobin or BPN. MN also contained esterases that split methyl butyrate and ß-naphthyl acetate. The pH optimum with the latter was 7.4, and its hydrolysis was partially inhibited by DFP, fluoride, taurocholate, and eserine. PMN had low esterase activity; RBC had little or none. MN, but not PMN or RBC, contained a stable lipase with a pH optimum of 6.1 in maleate buffer. Protamine, NaCl, heat, p-chloromercuribenzoate, ethylenediamine tetraacetate, taurocholate, and DFP were inhibitory, but no appreciable activation occurred in the presence of heparin or serum. Thus it possessed some of the characteristics of Korn's lipoprotein lipase, but not others.

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