The aldo-keto reductase (AKR) superfamily: an update.

The aldo-keto reductases (AKRs) are one of three enzyme superfamilies encompassing a range of oxidoreductases. Members of the AKR superfamily are monomeric (alpha/beta)(8)-barrel proteins, about 320 amino acids in length, which bind NAD(P)(H) to metabolize an array of substrates. AKRs have been identified in vertebrates, invertebrates, plants, protozoa, fungi, eubacteria, and archaebacteria, implying that this is an ancient superfamily of enzymes. Earlier, in an attempt to clarify the confusion caused by multiple names for particular AKRs, we proposed a systematic and expandable nomenclature system to assign consistent designations to unique members of the AKR superfamily. Since then, the number of characterized AKRs has expanded to 105 proteins in 12 families. In addition, molecular cloning and genome sequencing projects have identified 125 potential AKR genes, many of which have no assigned function. The nomenclature system for the AKR superfamily is accepted by the Human Genome Project. Using the earlier described nomenclature system, we now provide an updated listing of AKRs and potential superfamily members.