Tissues diurnal cycle

There are thus three main types of tissues to be conradered those which show no mitotic activity at any time, those which show diurnal cycles for the reasons stated above, and those which continue to undergo a maximum rate of cell division even in experimental conditions of acute starvation. The first group need not concern us here, since they appear to be firmly under that inherent mitotic control already mentioned. The... 

The next stage in mevalonate formation, involves two reductions of HMG-CoA by HMG-CoA reductase using NADPH. The enzyme is membrane-bound and is widely distributed in many tissues. It is a highly regulated enzyme with a short half-life (T1/2 about 3 hours) and its activity is often considered to control the overall rate of cholesterol biosynthesis. Even in a normal diurnal cycle the activity of the reductase will vary about tenfold. 

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