Effects of lipid removal on the molecular size and kinetic properties of bovine plasma arylesterase

Abstract

A purified arylesterase preparation from bovine plasma was characterized to the extent that it has a partial specific volume of 0.91ml/g and an apparent z-average molecular weight of 440000. The relatively large magnitude of the former reflects the presence of phospholipids, cholesterol, triglycerides and β-carotene, the last-named being responsible for the pronounced yellow colour of the preparation. Removal of the lipid material is accompanied by a decrease in the apparent z-average molecular weight to 120000, the size of the smallest species detected by high-speed sedimentation equilibrium being in the vicinity of 70000 daltons: denaturation of the lipid-free preparation with 6m-guanidine hydrochloride caused essentially complete breakdown into subunits of this size. In kinetic studies on the enzyme the maximal velocity for the hydrolysis of phenyl acetate was found to increase by 60% on addition of 1 mm-Ca2+, with the Km showing a concomitant decrease from 6.6 to 2.1 mm. Removal of lipid had no detectable effect on Vmax. or Km in either the presence or the absence of Ca2+. It is concluded that the bovine plasma arylesterase preparation is either a lipoprotein or an enzyme–lipoprotein complex with properties very similar to those of the α1-lipoprotein or high-density lipoprotein (HDL2) fraction of serum.