Enkephalin in bovine adrenal gland: Multiple molecular forms of [met5]-enkephalin immunoreactive peptides

The existence and heterogeneity of enkephalin-like substances in bovine adrenal glands has been demonstrated using enkephalin radioimmunoassays. Both [met5]- and [leu5]-enkephalins were found to be highly enriched in the medulla i.e. 6.65 pmol [met5]-enkephalin/mg protein but only a very small amount, 0.041 pmol [met5]-enkephalin/mg protein, was detected in the cortex. In addition to the pentapeptide, [met5]-enkephalin, a large proportion of the total [met5]-enkephalin immunoreactive substance was found by gel filtration chromatography to be present in two high molecular weight forms. Determination of the true concentration of the high molecular weight forms, however, awaits purification and use of a proper standard. The existence of the different molecular forms of [met5]-enkephalin immunoreactive substances was substantiated by immunoabsorption experiment and gel filtration chromatography with 6 M guanidine. The two high molecular weight immunoreactive substances were not assayable by the radioreceptor binding assay; however, when trypsinized, they released a product which was capable of binding to the opiate receptor and was indistinguishable from the pentapeptide. [met5]-enkephalin, with respect to its size and thin layer Chromatographic characteristic. Whether the high molecular weight [met5]-enkephalin immunoreactive substances can function as precursors of [met5]-enkephalin in adrenal glands remains to be determined.