Solubilization and Partial Purification of Neonatally Expressed Rat Hepatic Microsomal Monoacylglycerol Acyltransferase

Rat hepatic microsomal monoacyiglycerol acyltransferase (MGAT) is a developmentally expressed enzyme that catalyzes the formation of sn-1,2-diacylglycerol from sn-2-monoacylglycerol and fatty acyl-CoA. Treatment of suckling rat liver microsomes with various detergents showed that 0.3% Triton X-100, a nonionic detergent, solubilized a maximum amount of both protein (66%) and MGAT activity (56%). After solubilization with Triton X-100, MGAT was then purified 205-fold by sequential chromatography on QAE-Sephadex, CM-Sepharose (Fast Flow), and hydroxylapatite. Addition of phospholipids to the reaction mixture stimulated the purified enzyme activity more than 1.8-fold. sn-1,2-DiC18:1-glycerol activated purified MGAT activity. Purified MGAT activity was specific for sn-2-monoacylglycerol; the activity with rac-1-monoC18:1-glycerol and rac-1-and sn-2-monoCl8:1-glycerol ethers was less than 4% of the activity with sn-2-monoC18:1-glycerol. The purified MGAT had an isoelectric point of 9.7. The apparent Km and Vmax values of the purified enzyme for sn-2-monoC18:1-glycerol were 21 μM and 1036 nmol/min/mg, respectively. The apparent Km value for palmitoyl-CoA was 6.5 pM. Purified MGAT activity acylated sn-2-monoC18:2-glycerol and sn-2-monoC18:3-glycerol in preference to sn-2-monoC18:1-glycerol, consistent with a role for the monoacylglycerol pathway in retaining essential fatty acids.