Properties of Solubilized Microsomal Lipase from Germinating Brassica napus

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Abstract
Lipase (triacylglycerol acylhydrolase [EC 3.1.1.3.]) was extracted from the microsomal fraction of cotyledons of dark grown seedlings of Canola (Brassica napus L. cv Westar) by treatment with Triton X-100. The enzyme was partially purified by chromatography on Sephacryl S-300 and DEAE Bio-Gel and was stable when stored at -20.degree.C in 50% (v/v) glycerol. The lipase aggregated readily but the distribution of species present in solution could be controlled by nonionic detergents. A species with a apparent Mr of about 250,000 was obtained by gel filtration chromatography in the presence of 1% (v/v) Triton X-100. Lipase activity was optimal near neutral pH, and the reaction approached maximum velocity at a concentration of 0.5 to 1 millimolar emulsified triolein. The reaction rate responded linearly to temperature up to about 40.degree.C and the hydrolytic process had an activation energy of 18 kilocalories per mole. Microsomal lipase lost about 20% and 80% activity when heat-treated for 1 hour at 40.degree.C and 60.degree.C, respectively. At appropriate concentrations, the detergents Triton X-100, n-octyl-.beta.-D-glucopyranoside, (3-[3-cholamidopropyl-O-dimethylammonio]-1-propanesulfonate, cetyl trimethylammonium bromide, and sodium dodecyl sulfate all inhibited lipase activity. n-Octyl-.beta.-D-glucopyranoside, however, was stimulatory in the 2 to 8 millimolar concentration range. The inhibitory effects of Triton X-100 were reversible.