Radioactive human β-endorphin (β(h)-EP), tritiated at tyrosine-1 or -27 of both, with specific activity of 50 or 100 Ci/nmole, was prepared from its synthetic iodinated analogues by catalytic exchange. The tritiated peptide was shown to be homogeneous in CMC and partition chromatographic behavior as well as in isoelectric focusing. In addition, it possesses full biological activity in the guinea pig ileum assay. Tritiated analogs (≃ 0.3 μg), with or without added cold β(h)-EP (150 μg), were injected intravenously. Blood samples were taken at various times from a carotid cannula. Three to five animals were used for each experiment. In the rat, radioactivity had an initial half-life of 2.0 minutes, while in the rabbit the half-life was 4.5 minutes. These values were identical with or without added cold β-EP. After 15 minutes, radioactivity in the serum dropped much more slowly in both rat and rabbit, having a half-life of ≃60 minutes in the presence of cold β(h)-EP, and ≃6.0 hours in its absence. Injection of tritiated β(h)-EP also gave rise to radioactivity in the brain. The possibility of β(h)-EP uptake into the brain will be discussed.
