Apparent Co-Sequestration of Immunoreactive Corticotropin, α-Melanot opin, and γ-Lipotropin in Hypothalamic Granules

Abstract: In the present study, the question of whether immunoreactive α-melanotropin (α-MSH₁), corticotropin (ACTH₁), and β-melanotropin (β-MSH₁) are co-sequestered in hypothalamic granules of adult male rats was addressed. When a 900 ×g supernatant fluid prepared from a hypothalamic homogenate was fractionated on continuous sucrose density gradients under non-equilibrium Conditions, two populations of particles containing α-MSH₁, ACTH₁, or β-MSH₁ Were observed. However, when fractionated under equilibrium conditions, the two populations of particles containing α-MSH₁ ACTH₁, or β-MSH₁ were recovered as a single band. This sedimentation characteristic indicates that the particles containing a given peptide differ in size but are similar in density. In their sedimentation, the small particles containing α-MSH₁, ACTH₁, and β-MSH₁ are indistinguishable from granules containing α-MSH₁, whereas the large particles containing α-MSH₁ (ACTH₁, and β-MSH₁ are indistinguishable from synaptosomes containing α-MSH₁, β-MSH₁ had an apparent molecular weight (M.W.) of about 5,000, which is similar to that of γ-lipotropin. ACTH₁ was comprised of three species of molecules: big (M.W. ≥ 10,000), 5.7K (M.W. ≌ 5,700), and 4.5K (M.W. ≌ 4,500). Big ACTH was the predominant and 5.7K ACTH the minor component of ACTH₁ present in granules as well as in synaptosomes. These results are suggestive that α-MSH, ACTH and its precursors, and γ-lipotropin are co-sequestered in hypothalamic granules.