It is now generally accepted that a major risk factor for the development of coronary heart disease is an elevated concentration of plasma cholesterol, especially low density lipoprotein (LDL) cholesterol. The objective is to decrease excess levels of cholesterol to an amount consistent with maintenance of normal body function. Cholesterol is biosynthesized in a series of more than 25 separate enzymatic reactions that initially involves 3 successive condensations of acetyl-CoA units to form a 6-carbon compound, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA). This is reduced to mevalonate and then converted in a series of reactions to the isoprenes that are building blocks of squalene, the immediate precursor to sterols, which cyclizes to lanosterol (a methylated sterol) and further metabolized to cholesterol. A number of early attempts to block the synthesis of cholesterol resulted in agents that inhibited late in the biosynthetic pathway between lanosterol and cholesterol. A major rate limiting step in the pathway is at the level of the microsomal enzyme which catalyzes the conversion of HMG CoA to mevalonic acd and which has been considered to be a prime target for pharmacologic intervention for several years.
HMG CoA reductase occurs early in the biosynthetic pathway and is among the first committed steps to cholesterol formulation. Inhibition of this enzyme could lead to accumulation of HMG CoA, a water-soluble intermediate that is then capable of being readily metabolized to simpler molecules. This inhibition of reductase would lead to accumulation of lipophylic intermediates having a formal sterol ring.
Lovastatin is the first specific inhibitor of HMG CoA reductase to receive approval for the treatment of hypercholesterolemia. The first breakthrough in efforts to find a potent, specific, competitive inhibitor of HMG CoA reductase occurred in 1976 when Endo et al reported discovery of mevastatin, a highly functionalized fungal metabolite, isolated from cultures of Penicillium citrium. Mevastatin was demonstrated to be an unusually potent inhibitor of the target enzyme and of cholesterol biosynthesis. Subsequent to the first reports describing mevastatin, efforts were initiated to search for other naturally occurring inhibitors oh HMG CoA reductase. This led to the discovery of a novel fungal metabolite – Lovastatin. The structure of Lovastatin was determined to be different from that of mevastatin by the presence of a 6 alphamethyl group in the hexahydronaphthalene ring.
Key points from the study of the Biosynthesis of Lovastatin :-
Lovastatin is comprised of 2 polyketide chains derived from acetate that are 8- and 4- carbons long coupled in head to tail fashion.
6 alphamethyl group and the methyl group on the 4-carbon side chain are derived from the methyl group of methionine, and
6 alphamethyl group is added before closure of the rings.
This implies that lovastatin is a unique compound synthesized by A. terreus and that mevastatin is not an intermediate in its fornmation.