An alteration in the kinetic properties of a cholesterogenic enzyme in fowlpox virus infection

Abstract

Cholesterogenesis in the chorioallantoic membrane (CAM) of the chick embryo during fowlpox virus infection has been studied by a variety of techniques. Analysis of the sterol content of the CAM epithelium revealed an elevation in the sterol content on the fourth day postinfection, but not on the third day. The increase was due primarily to the accumulation of sterol ester. That the increased level of sterols is due to an increase in synthetic rate is suggested by the observation that the activity of the enzyme usually considered to be rate limiting in cholesterogenesis, 3-hydroxy,3-methylglutaryl coenzyme A (HMG-CoA) reductase, becomes elevated above control values at about this same time period, i.e., between 3 and 4 days postinfection. The ability to synthesize sterol from labeled mevalonate is lower in the infected tissue than in the control. These data indicate that the HMG-CoA reductase reaction may be a primary site of action in the stimulation of cholesterogenesis in fowlpox virus infection. An examination of the kinetic properties of the HMG-CoA reductase from infected CAM epithelium suggested that two forms of the enzyme are present in infected cells, one form identical to the control enzyme- and an altered form. This conclusion was based on the biphasic nature of the saturation curve of the enzyme for HMG-CoA. Other kinetic properties such as thermal denaturation and pH titration also revealed differences between the enzymes from control and infected tissues.