Angiotensin-converting enzyme: accumulation in medium from cultured endothelial cells.

The activity of angiotensin converting enzyme has been measured in endothelial cells cultured from hog aorta, and in serum-free culture medium taken from both endothelial cells and smooth muscle cells. Endothelial cells maintained in medium containing 20% fetal calf serum contained 43 pmol/min/10⁶ cells of converting enzyme activity; freshly collected cells contained 518 pmol/min/10⁶ cells. Endothelial cells held in serum-free medium release this activity into the medium in amounts up to 40 times that associated with the cells; at the same time the activity associated with the cells rises 2 fold. The rise in cell-associated activity and the appearance of activity in the medium are both blocked by cycloheximide. These observations provide direct evidence that endothelial cells can produce excess angiotensin-converting enzyme and release it in active form, and thus serve as the source of circulating converting enzyme activity.