The terminal enzymes of cholesterol synthesis, DHCR24 and DHCR7, interact physically and functionally

Cholesterol is essential to human health, and its levels are tightly regulated by a balance of synthesis, uptake, and efflux. Cholesterol synthesis requires the actions of more than twenty Enzymes to reach the final product, through two alternate pathways. Here we describe a physical and functional interaction between the two terminal Enzymes. 24-Dehydrocholesterol reductase (DHCR24) and 7-dehydrocholesterol reductase (DHCR7) coimmunoprecipitate, and when the DHCR24 gene is knocked down by siRNA, DHCR7 activity is also ablated. Conversely, overexpression of DHCR24 enhances DHCR7 activity, but only when a functional form of DHCR24 is used. DHCR7 is important for both Cholesterol and vitamin D synthesis, and we have identified a novel layer of regulation, whereby its activity is controlled by DHCR24. This suggests the existence of a Cholesterol "metabolon", where Enzymes from the same metabolic pathway interact with each Other to provide a substrate channeling benefit. We predict that other Enzymes in Cholesterol synthesis may similarly interact, and this should be explored in future studies.