Oxysterols inhibit plant Sterol methyl transferase type-1 enzyme activity in vitro

Abstract: Sterols are important structural components of eukaryotic cell membranes, where they influence membrane fluidity and permeability. Sterols contain a four-ring structure with a hydrocarbon side-chain. The sterol composition and structure differs among plants, mammals and fungi. Mammals and fungi contain a single main sterol such as cholesterol or ergosterol, respectively. In contrast, plants contain a mixture of several sterols with varying side-chain length such as sitosterol, stigmasterol and 24-methyl cholesterol. Side-chain length is regulated by the two enzymes, Sterol methyl transferase type-1 (SMT1) and type-2 (SMT2) which increase side-chain length by a methyl (SMT1) or ethyl (SMT2) group. Thus, regulation of SMT enzyme activity is crucial for sterol composition in plants. This project considers to what extent oxysterols, oxidized sterol forms, can regulate SMT1 activity. SMT1 activity was assayed in microsome preparations from three plant species, and different oxysterols were added to the enzyme reaction to investigate the effect on enzyme activity. Results showed that oxysterols inhibited SMT1 activity in all plant species tested. The inhibitory effect was stronger for side-chain oxysterols (24-OH and 25-OH cholesterol) than for ring hydroxylated ones (7β-OH cholesterol). The results suggest that oxysterols have an inhibitory effect on SMT1 activity in plants, and may be part of a feed-back regulation of sterol composition.