Lysophosphatidylcholine metabolism in the rabbit heart. Characterization of metabolic pathways and partial purification of myocardial lysophospholipase-transacylase

Abstract

Metabolism of lysophosphatidylcholine (LPC), recently implicated in arrhythmogenesis, was characterized in rabbit ventricular homogenates. Activities of four enzymatic pathways were distinguishable after subcellular fractionation and DEAE-Sephacel chromatography including microsomal lysophospholipase, microsomal acyl coenzyme A/LPC acyltransferase, cytosolic lysophospholipase, and cytosolic lysophospholipase-transacylase. Microsomal lysophospholipase activity was attenuated 81% by acidosis comparable to that in ischemic myocardium (pH 6.5) and was inhibited by substrate. LPC acyltransferase was identified in the microsomal fraction based on CoA-dependent phosphatidyl choline synthesis, the positional specificity of acylation of LPC, and identical reaction velocities with both of its labeled co-substrates. LPC acyltransferase had a Vmax of 5.1 nmol/mg/min, a broad pH optimum centered at pH 7, and an apparent Km for LPC and palmitoyl-CoA of 14 microM and 7 microM. Cytosolic lysophospholipase was separated from lysophospholipase-transacylase by DEAE-Sephacel chromatography and distinguished from microsomal lysophospholipase by its broad pH activity curve, Michaelis-Menten kinetics (Vmax = 9.5 nmol/mg/min, Km = 7.5 microM), and lack of substrate inhibition. Lysophospholipase-transacylase was identified in the cytosolic fraction by CoA-independent phosphatidyl choline synthesis and purified 4885-fold from homogenate by ammonium sulfate precipitation, DEAE-Sephacel, hydroxylapatite, gel filtration, and polylysine chromatography. The partially purified enzyme had a transacylase/lysophospholipase activity ratio of 0.6, and transacylation of LPC was prominent at submicellar concentrations of substrate.