1992 Volume 40 Issue 7 Pages 1755-1760
Dramatic changes in the activities of squalene-2, 3-epoxide : cycloartenol cyclase and β-amyrin cyclase were observed in germinating pea seeds. By taking advantage of this phenomenon, the two cyclases were purified from pea seedlings. The cyclases were purified to homogeneity by solubilization with Triton X-100, chromatography on hydroxylapatite and diethylaminoethyl (DEAE)-cellulose, isoelectric focusing and gel filtration. Cycloartenol cyclase was purified 471-fold to a specific activity of 167 pkat/mg protein, while β-amyrin cyclase was purified 4290-fold to a specific activity of 28 pkat/mg protein. They each showed a single band on sodium dodecyl sulfate polyacrylamide gel electrophoresis with a molecular mass of 55 and 35 kilodaltons (kDa), respectively. The apparent Km values for (3S)-squalene-2, 3-epoxide were estimated to be 25 and 50 μM, respectively. The cyclases required Triton X-100 or deoxycholate for their highest activity and each showed a broad pH optimum within the range of pH 6.5-7.5. Inhibition by p-chloromercuribenzene sulfonic acid and N-ethylmaleimide suggested involvement of an SH group at the active site of each enzyme.
