A cytochrome P450 monooxygenase responsible for the C-22 hydroxylation step in the Paris polyphylla steroidal saponin biosynthesis pathway

Paris polyphyllaSmith var. yunnanensis (Franch.) Hara is the main source of Paridis Rhizoma, a common Chinese herbal medicine. Its major active components are steroidal saponins such as polyphyllin I, which have antitumor and immuno-stimulating properties as well as other pharmacological activities. In this study, we cloned the CYP450 gene PpCYP90B27, which encodes an enzyme that catalyzes the formation of 22(R)-hydroxycholesterol from cholesterol. This CYP450 enzyme from P. polyphylla is responsible for the hydroxylation in the first step downstream from cholesterol in the steroidal saponin pathway, and is involved in a hitherto unknown biosynthesis pathway of bioactive ingredients polyphyllins. To our best knowledge, this is the first cloned CYP450 enzyme gene from plants in the steroidal saponin pathway.253