Purification and characterization of hydroxycinnamoyl D-glucose. Quinate hydroxycinnamoyl transferase in the root of sweet potato, Ipomoea batatas Lam

We have previously proposed a chlorogenic acid biosynthetic pathway which involves a transesterification reaction between hydroxycinnamoyl D-glucose and D-quinic acid. The proposed pathway was based on tracer experimental results (Kojima, M., and Uritani, I. (1972) Plant Cell Physiol. 13, 311-319). The enzyme that catalyzes the above reaction has been purified 160-fold from sweet potato root (Ipomoea batatas Lam.) and characterized. The purified enzyme yielded one band of 26,000 daltons on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and its molecular weight was estimated to be 25,000 by gel filtration chromatography. Therefore, the enzyme seems to consist of a single polypeptide of 25,000-26,000 daltons. The isoelectric point of the enzyme was 8.6. The optimum pH of the enzyme reaction was 6.0. The enzyme did not require any metal for activity and showed a broad substrate specificity toward hydroxycinnamoyl D-glucose as donors. The Km and Vmax values were 3.7 mM and 8.5 units/mg of protein for t-cinnamoyl D-glucose, 3.9 mM and 15.1 units/mg of protein for p-coumaroyl D-glucose, and 14.3 mM and 38.1 units/mg of protein for caffeoyl D-glucose. The enzyme showed a strict substrate specificity toward D-quinic acid-related compounds as acceptors; the Km and Vmax values were 16.7 mM and 15.1 units/mg of protein for D-quinic acid, 250 mM and 19.0 units/mg of protein for shikimic acid, and there was no activity with either L-malic acid or meso-tartaric acid. The enzyme activity changed in a manner suggesting its involvement in chlorogenic acid biosynthesis during incubation of sliced sweet potato root tissues.