Vitamin C, a reducing agent and antioxidant, is a cofactor in reactions cata-lyzed by Cu + -dependent monooxygenases and Fe 2+ -dependent dioxygenas-es. It is synthesized, in vertebrates having this capacity, fromd-glucuronate. The latter is formed through direct hydrolysis of uridine diphosphate (UDP)-glucuronate by enzyme(s) bound to the endoplasmic reticulum mem-brane, sharing many properties with, and most likely identical to, UDP-glucuronosyltransferases.