Modification of Rhizopus delemar Lipase by Its Binding with Phospholipids1

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Abstract
Rhizopus delemar (ATCC 34612) lipase was modified by phospholipid (PL)-treatment so as to enhance its activity on lipoprotein. In order to detect change in lipase conformation in the modified state, a preliminary experiment was performed to remove PL from the PL-treated lipase solution which included PL nonessential to enhancement of lipoprotein lipase (LPL) activity. It was found that treatment with a mixture of isopropyl ether: n-butanol (3 : 1) was suitable for this purpose because of the stability of the enzyme. Changes in isoelectric point and α-helical content of lipase caused by PL-treatment were studied by means of isoelectric focusing and circular dichroism spectrum. The isoelectric point of lipase was found to shift to the acidic side on its binding with phosphatidylcholine (PC) or cardiolipin (CL). The circular dichroism spectra of the original lipase and PL-treated lipases indicated that the α-helical content of lipase decreased on its binding with PL. In CL-bound lipase, which was more greatly enhanced as to LPL activity than was PC-bound lipase, α-helical content was decreased to a larger extent than that of PC-treated lipase.