乳清酸经乳清苷酸合成UMP HN CH HN CH HN CH 0 PRPP PP, 0-H+02 Orotate 0-P0CH20 oe→0-00 phosphoribosyl decarboxylase ONH transferase Orotate Orotidylate Uridylate (UMP乳清酸经乳清苷酸合成UMP