Enzymic Synthesis of Mevalonic Acid in Plants
The biosynthesis of mevalonic acid (“MVA”), starting from acetyl-CoA (“Ac-CoA”), requires the action of three enzymes: a) acetoacetyl-CoA thiolase (“AACT”, EC 188.8.131.52), b) 3-hydroxy-3-methylglutaryl-CoA synthase (“HMGS”, EC 184.108.40.206), and c) HMG-CoA reductase (“HMGR”, EC 220.127.116.11). Our recent research has mainly centered around the purification and characterization of membrane-bound HMGR from radish seedlings (1–3). HMGR activity is commonly regarded as playing an important role in the regulation of substrate flux from acetate to the various isoprenoid endproducts, mainly of sterols (cf. 2,6). It is well documented that regulation of mammalian HMGR is mediated by rapid processes both at the translational and post-translational level (cf. 10). Regulation of mammalian HMGS appears to parallel that of HMGR (cf. 5). Similar processes can be expected to occur in plant cells. A thorough study in this direction requires a profound knowledge of the properties of the enzymes involved in the formation of HMG-CoA. This substrate, besides being reduced by HMGR to yield MVA, can also be utilized by HMG-CoA lyase (“HMGL”, EC 18.104.22.168) to form acetoacetate, Ac-CoA and HS-CoA, an important reaction which in mammalian cells (mitochondria) contributes to the HMG-CoA cycle and hence to the formation of ketone bodies.
KeywordsAmmonium Sulfate Ketone Body Apparent Molecular Mass Mevalonic Acid Isoprenoid Pathway
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