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°C with fractions of rat brain. The amount of intact tetrapeptide remaining was determined by HPLC. By 3 min, most of the Tyr‐MIF‐1 was degraded. Because similar amounts of [3H]Pro and [3H]Tyr appeared after incubation of the Tyr‐MIF‐1 peptides in brain homogenate, even as early as 30 s, examination of only this crude preparation would misleadingly indicate that Tyr‐MIF‐1 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 [3H]Pro, with accompanying accumulation of MIF‐1. Addition of excess MIF‐1 to the mitochondrial fraction completely suppressed the formation of MIF‐1 and more than doubled the amount of Tyr‐MIF‐1 remaining intact. When Tyr‐MIF‐1 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‐1, not at the position of Tyr‐Pro. The results indicate that Tyr‐MIF‐1 can serve as a precursor of MIF‐1 in brain mitochondria, an effect not evident when crude brain homogenate is used.