The mouse epididymis: A site of strong and constitutive expression of the tryptophan metabolizing enzyme indoleamine 2,3-dioxygenase (IDO)

.Some twenty-five years ago it was reported for the first time that indoleamine 2,3-dioxygenase (IDO) activity was particularly important in rodent epididymis protein extracts. Several years ago, it was also shown that the epididymal expression of IDO was constitutive and not driven by the interferon-gamma, inflammatory cytokine, as is the case in most of the other tissues tested. Since then, the epididymal expression of IDO has not been investigated further. Recently, we have reported a more detailed study showing that indeed the mouse epididymis expresses both IDO transcript and protein at a high level while this is not the case for the other tryptophan-catabolizing enzyme, TDO. On the basis of the different functions that have been assigned to IDO we discuss here the putative roles of IDO expression as well as tryptophan metabolism on the physiology of the mammalian epididymis.