Lipolytic Enzymes in Bovine Thyroid Tissue I. Subcellular localization, purification and characterization of acid phospholipase A₁

Abstract

In mammalian cells the catabolism of membrane phosphoglycerides proceeds probably entirely through a deacylation pathway catalysed by phospholipase A and lysophospholipase (Wise & Elwyn, 1965). In the initial attack of diacylphosphoglycerides by phospholipase A two enzymatic activities with different positional specificities have been distinguished : phospholipase A₁ (phosphatidate 1-acyl hydrolase EN 3.1.1.32) and phospholipase A₂ (phosphatidate 2-acyl hydrolase EN 3.1.1.4) (Van Deenen & De Haas, 1966). Studies on these intracellular phospho-lipases were mainly concerned with their subcellular localization. Only occasionally more detailed enzymatic investigations have been conducted on them, in contrast to export phospholipases e.g. from snake venom, bee venom and porcine pancreas, which have been extensively investigated (Brockerhoff & Jensen, 1974a).

In a previous paper (De Wolf et al., 1976a), the presence of phospholipase A₁ and phospholipase A₂ activities in bovine thyroid was demonstrated, using 1 -[9,10-³H] stearoyl-2-[1 ¹⁴C] linoleyl-sn-glycero-3-phosphocholine as a substrate. Optimal activity was observed in both instances at pH 4. Addition of the anionic detergent sodium taurocholate increased the A₂ type activity and decreased the A₁ type activity suggesting the presence of different enzymes. The lack of influence of Ca²⁺-ions and EDTA and the acid pH optima could suggest lysosomal localization.

In this paper the subcellular distribution of both acid phospholipase activities is described as well as a purification scheme for phospholipase A₁. Some characteristics of the purified enzyme preparation are discussed.