Inhibition of bone resorption in vitro by human enkephalinase (EC 3.4.24.11), a neutral metalloendopeptidase.

Bone metabolism is regulated by a wide variety of both circulating and locally produced peptides. The activity of such agents must be regulated, and one potential regulating mechanism is the inactivation of these peptides by locally produced proteolytic enzymes. One candidate for such a class of enzymes is enkephalinase (EC 2.3.24.11), a membrane-bound neutral metalloendopeptidase that inhibits the activity of a range of biologically active peptides, including interleukin-1 (IL-1), a potent bone-resorbing agent. In this study, we examined the effects of human enkephalinase on bone resorption in cultures of fetal rat long bones. We found that partially purified and highly purified enkephalinase inhibited bone resorption stimulated by parathyroid hormone (PTH) and IL-1 alpha. The effects on PTH-stimulated resorption were reversible, but enkephalinase did not inhibit prestimulated resorption. Enkephalinase also inhibited resorption induced by the nonpeptide stimulators 1,25-(OH)2D3, retinoic acid, and prostaglandin E2 (PGE2). In addition, preliminary studies confirmed a previous report of the presence of an enkephalinase-like activity in osteoblast-like osteosarcoma cells. These data are consistent with the hypothesis that proteolytic enzymes, such as enkephalinase, may play a role in the local regulation of bone resorption.

Effect of human endopeptidase 24.11 ("enkephalinase") on IL-1-induced thymocyte proliferation activity.

Endopeptidase 24.11 (enkephalinase) is a membrane bound protease involved in the degradation of neuropeptides and hormones. Its presence on cells of the thymus and lymph nodes suggests a possible role in the inactivation of immune system mediators. IL-1 (both purified IL-1 beta and an IL-1-rich supernatant) bioactivity, as measured in the thymocyte proliferation assay, was found to disappear upon incubation with endopeptidase 24.11. This inactivation was dependent on both incubation time and enzyme concentration. IL-1 beta was protected by the presence in the incubation medium of phosphoramidon, a specific inhibitor of endopeptidase 24.11. After incubation of IL-1-rich supernatant with the enzyme, the thymocyte proliferation activity could be restored by adding purified IL-1 beta to the samples, indicating that neither the enzyme nor the buffer had any toxic effect on thymocyte proliferation. In the same experimental conditions, IL-2 activity was not destroyed by endopeptidase 24.11.