Tissue kallikrein in pituitary and brain

The glandular or tissue kallikreins are a subgroup of serine proteases, some of which have been shown to be involved in the processing of polypeptides to their bioactive forms. Following the demonstration that tonin, a member of the rat kallikrein gene family, could cleave proopiomelanocortin in vitro to yield ACTH, attention focused on the possible presence of kallikrein, tonin, and/or other family members in the pituitary. Kallikrein-like enzyme activity, immunoreactivity, and gene expression has since been demonstrated in the rat anterior pituitary, where it is estrogen-dependent, and in the neurointermediate lobe, where levels are regulated by dopaminergic mechanisms. These patterns of regulation are similar to that of prolactin in the anterior pituitary and proopiomelanocortin in the neurointermediate lobe. Immunoreactive kallikrein has been localized to the lactotrope in the anterior pituitary; the suggested localization to the melanotrope in the neurointermediate lobe has not yet been definitively demonstrated. The estrogen-dependent kallikrein gene expressed in the rat anterior pituitary and neurointermediate lobe has been shown to be true glandular kallikrein. The potential role(s) of true kallikrein or other kallikrein family members in the pituitary may be in the regulation of secretion and/or processing of pituitary hormones (e.g., prolactin and proopiomelanocortin). Kallikrein is also found in the hypothalamus and, to a lesser extent, other regions of the brain where it may be involved in processing neuropeptides. It could also be involved in the regulation of cerebral circulation.