Melanocyte-stimulating hormone release-inhibiting factor-1 (MIF-1) can be formed from Tyr-MIF-1 in brain mitochondria but not in brain homogenate.

Two samples of the peptide tyrosine-melanocyte-stimulating hormone release-inhibiting factor-1 (Tyr-MIF-1; Tyr-Pro-Leu-Gly-NH2) were tritiated on different amino acids (Tyr or Pro) and incubated together at 37 degrees C with fractions of rat brain. The amount of intact tetrapeptide remaining was determined by HPLC. By 3 min, most of the Tyr-MIF-l was degraded. Because similar amounts of [H-3]Pro and [H-3]Tyr appeared after incubation of the Tyr-MIF-l peptides in brain homogenate, even as early as 30 s, examination of only this crude preparation would misleadingly indicate that Tyr-MIF-l is not a precursor of melanocyte-stimulating hormone release-inhibiting factor-1 (MIF-1; Pro-Leu-Gly-NH2) in brain tissue. However, incubation of the mitochondrial fractions of brain under the same conditions resulted in more than three times as much [3H]Tyr being formed as [H-3]Pro, with accompanying accumulation of MIF-1. Addition of excess MIF-1 to the mitochondrial fraction completely suppressed the formation of MIF-I and more than doubled the amount of Tyr-MIF-l remaining intact. When Tyr-MIF-l tritiated only on the Tyr was added to the mitochondrial fraction, the main peaks of radioactivity appeared only at the positions of Tyr and Tyr-MIF-l, not at the position of Tyr-Pro. The results indicate that Tyr-MIF-l can serve as a precursor of MIF-I in brain mitochondria, an effect not evident when crude brain homogenate is used.