Isolation and purification of UDP-glucuronosyltransferases.

UDPGTs are members of a class of enzymes located in the endoplasmic reticulum and are encoded by a multigene family. These proteins are responsible for the glucuronidation of hundreds of xenobiotics of many chemical classes and many endogenous substances such as steroid hormones, bile acids, and bilirubin. There are a number of UDPGTs which have been identified by purification and characterization studies and a significant number which have been characterized by expression of cDNAs. On the basis of the primary structures elucidated they appear to have marked similarities (5) and are highly conserved. However, key differences in their functional properties appear to depend primarily on differences in amino acid sequences at or about the NH2-terminal area of the protein (5). Many of the UDPGTs have an extraordinarily broad substrate specificity; a few, however, are relatively specific for a given class of substrate (morphine, DT-1 UDPGTs). This places a burden on investigators to clearly identify which substrate and how many UDPGTs will be involved in any analysis of rates of glucuronidation in microsomal preparations. Caution should also be advised for extrapolation of data from hepatic microsomes of experimental animals to human hepatic microsomal preparations because human liver microsomes possess UDPGTs which are qualitatively different and, in certain cases, UDPGTs are present in human liver which are not present in lower animals.