Maintenance of respiratory control by beef heart mitochondria incubated at 25 degrees C: response to protective agents and to protective agents and to prior stress.

Lysophospholipase D (EC 3.1.4.-) activity was demonstrated in rat kidneys, intestines, lungs, testes, and liver. The liver enzyme was studied in greatest detail and its labeled products were identified by chemical and Chromatographic techniques. This enzyme hydrolyzes 1-1-¹⁴C]hexadecyl-sn-glycero-3-phosphoethanolamine and 1-1-¹⁴C]hexadecyl-sn-glycero-3-phosphocholine to yield 1-1-¹⁴C]hexadecyl-sn-glycero-3-phosphate; the initial product is subsequently dephosphorylated by a phosphohydrolase in microsomes to form 1-1-¹⁴C]hexadecyl-sn-glycerol. The possibility that phospholipase C and a phosphotransferase were responsible for the formation of 1-1-¹⁴C]hexadecyl-sn-glycero-3-phosphate was ruled out. Neither 1-1-¹⁴C]hexadecyl-2-acyl-sn-glycero-3-phosphoethanolamine nor 1-1-¹⁴C]hexadecyl-2-acyl-sn-glycero-3-phosphocholine was hydrolyzed. The enzyme requires Mg²⁺, is inhibited by Ca²⁺, and is stimulated by high salt concentrations; it is localized in the microsomal fraction and has a pH optimum between 7.0 and 7.6. Inhibition by sulfhydryl reagents and protection by glutathione and dithiothreitol suggest that a sulfhydryl group is required for activity. The enzyme is inhibited by detergents and by organic solvent extraction. It appears to be tightly bound to the microsomes, since repeated freeze-thawing or sonication did not release the activity, and trypsin digestion (either in the presence or in the absence of 0.04% deoxycholate) did not destroy the activity. Lysophospholipase D was previously known to occur only in brain (R. L. Wykle and J. M. Schremmer, 1974, J. Biol. Chem., 249, 1742–1746).