Thermal Responsiveness of 3‐Hydroxy‐3‐methylglutaryl‐Coenzyme‐A Reductase and Acetyl‐Coenzyme‐A Carboxylase in Neoplastic Guinea Pig Lymphocytes (L 2 C)

Hydroxymethylglutaryl‐CoA reductase and acetyl‐CoA carboxylase, rate‐limiting enzymes of cholesterol and fatty acid biosynthesis, respectively, do not show equivalent thermal responsiveness in leukemic L 2 C guinea‐pig lymphocytes. Both enzymes exhibit Arrhenius plots characteristic of membrane‐association but hydroxymethylglutaryl‐CoA reductase yields a single slope, compared with three slopes, crossing at 24°C and 12°C for acetyl‐CoA carboxylase. Preincubation with phosphatidylcholine liposomes before enzyme assay does not modify the thermal behavior of acetyl‐CoA carboxylase but introduces a discontinuity at 24 C in the Arrhenius diagramin of hydroxymethylglutaryl‐CoA reductase, conferring to this enzyme an activation energy close to that of normal cells. The Arrhenius plot for fatty acid biosynthesis in L 2 C cells parallels the thermotropism of the acetyl‐CoA carboxylase, with two discontinuities at 25 and 15 °C. However, cholesterol biosynthesis shows a discontinuity at 24.6 °C, whereas hydroxymethylglutaryl‐CoA reductase activity does not. The different thermal behavior of hydroxymethylglutaryl‐CoA reductase and acetyl‐CoA carboxylase is discussed in terms of lipid heterogeneities in membrane enzyme environments, i.e. enzyme localization in dissimilar lipid domains. We propose several hypotheses to account for the lack of correlation between the thermotropic responses of hydroxymethylglutaryl‐CoA reductase activity and of cholesterol biosynthesis in L 2 C cells.