. "\"Catalysis of the formation of the C-22(23) double bond in the sterol side chain. An example reaction: 5,7,24(28)-ergostatrienol + O2 + NADPH = 5,7,22,24(28)-ergostatetraenol + 2 H2O + NADP+.\" [MetaCyc:RXN3O-227]" . . "C-22 sterol desaturase activity" . . . . . . . . . . . . . . .