The diurnal variation of hepatic HMG-CoA reductase activity is due to changes in the level of immunoreactive protein.

The diurnal variation in levels of hepatic HMG-CoA reductase immunoreactive protein and enzyme activity were determined in rats. Immunoreactive protein levels changed together with enzyme activity. Thus the catalytic efficiency of HMG-CoA reductase was not significantly changed. The data suggest that the diurnal variation in hepatic HMG-CoA reductase activity is due to changes in enzyme protein levels rather than changes in phosphorylation state of the enzyme, for example, which would cause changes in catalytic efficiency.