To elucidate the cellular location of mitochondrial malic enzyme in brain, immunocytochemical studies were performed. For this purpose, mitochondrial malic enzyme was purified to apparent homogeneity from bovine brain and used for the immunization of rabbits. Subjecting the antiserum to affinity purification on immobilized antigen as an absorbent yielded a purified immunoreactive antibody preparation, which was characterized by probing cytosolic and mitochondrial fractions of bovine and rat brain in western blotting. As neither crossreactivity with cytosolic malic enzyme nor immunoreactivity against other proteins could be observed, the antibody preparation was found suitable for immunocytochemistry. By using sections of perfusion-fixed rat brain, considerable resolution was achieved at the light-microscopic level. Distinct and specific staining of neurons was observed; in contrast, no staining of astrocytes and possibly unspecific staining within the nuclei of oligodendrocytes were obtained. From these data, it is concluded that mitochondrial malic enzyme is located in neurons; however, in astrocytes, the enzyme appears to be either lacking or present at a much lower level. A protective role against oxidative stress in neurons is proposed for mitochondrial malic enzyme.

Originally defined as a lymphokine inhibiting the random migration of macrophages, the macrophage migration inhibitory factor (MIF) is an important mediator of the host response to infection. Beyond its function as a classical cytokine, MIF is currently portrayed as a multifunctional protein with growth-regulating properties present in organ systems beyond immune cells. In previous studies, we detected substantial amounts of MIF in the rat epididymis and epididymal spermatozoa, where it appears to play a role during post-testicular sperm maturation and the acquisition of fertilization ability. To explore its presence in other species not yet examined in this respect, we extended the range of studies to the bull. Using a polyclonal antibody raised against MIF purified from bovine eye lenses, we detected MIF in the epithelium of the adult bovine epididymis with the basal cells representing a prominently stained cell type. A distinct accumulation of MIF at the apical cell pole of the epithelial cells and in membranous vesicles localized in the lumen of the epididymal duct was obvious. In the fetal bovine epididymis, we also detected MIF in the epithelium, whereas MIF accumulation was evident at the apical cell surface and in apical protrusions. By immunoelectron microscopy of the adult bovine epididymis, we localized MIF in apical protrusions of the epithelial cells and in luminal membrane-bound vesicles that were found in close proximity to sperm cells. Although the precise origin of the MIF-containing vesicles remains to be delineated, our morphological observations support the hypothesis that they become detached from the apical surface of the epididymal epithelial cells. Additionally, an association of MIF with the outer dense fibers of luminal spermatozoa was demonstrated. Data obtained in this study suggest MIF release by an apocrine secretion mode in the bovine epididymis. Furthermore, MIF localized in the basal cells of the epithelium and in the connective tissue could be responsible for regulating the migration of macrophages in order to avoid contact of immune cells with spermatozoa that carry a wide range of potent antigens.