Brain-specific expression of a cDNA encoding human mitochondrial creatine kinase. The brain-specific expression of a cDNA encoding human mitochondrial creatine kinase (hMi-CK) was investigated in transfected C2 mouse neuroblastoma cells and transgenic mice carrying the plasmid. hMi-CK transcripts of 6.0 kb were observed in the cortex and hippocampus of brain, while only minor amounts were detected in liver. We detected a small amount of hMi-CK protein in both extracts of brain and liver. However, the relative amount of hMi-CK protein was extremely low in brain, while it was high in liver. These findings suggest that the translational or post-translational regulation of hMi-CK protein expression could be tissue specific, but it was not clear whether this regulation was at the transcriptional or post-transcriptional level. Interestingly, we found a species-specific difference between mouse and human in the translational regulation of hMi-CK: the expression of human hMi-CK mRNA was detected in mouse brain, but not in human brain. Mouse and human hMi-CK proteins were not expressed in brain. The human hMi-CK protein was expressed in human myogenic HL-1 cells, which did not express mouse hMi-CK mRNA and protein. Immunohistochemical staining showed that the endogenous hMi-CK protein was expressed in pyramidal neurons of cerebral cortex, cerebellum, and hippocampus. In transgenic mice, the transgenic hMi-CK protein was expressed in cerebral cortex, but not in cerebellum and hippocampus. The human but not mouse hMi-CK gene was correctly expressed in brain of the transgenic mice, showing the translational and tissue-specific regulation of hMi-CK in a species-specific manner. These results suggest that regulatory regions involved in the tissue-specific expression and/or translational control of hMi-CK were present within the 5'-upstream region of the hMi-CK gene, especially in the hMi-CK gene of different species.