Enkephalinase and angiotensin converting enzyme activities in human venous and arterial plasma

Circulating opioids, particularly enkephalins, can act on specific receptors located on the neurovascular sympathetic junction. These peptides are quickly metabolized by enkephalinase and angiotensin converting enzyme (ACE). According to the parallel distribution of enkephalinase with opioid receptors in the rat brain, and its location in the vascular bed, putative differences of enkephalinase and ACE activity between arterial and venous plasma of the same subjects was researched. Venous enkephalinase activity was found to be greater than arterial activity. No arterovenous differences were present in ACE activity. The activities of both enzymes presented a positive correlation between venous and arterial plasma in the same subjects. The arterovenous difference in enkephalinase activity supports a release of the enzyme from microvessels.