The subcellular distribution of fumarase was investigated in the liver of various animals and in several tissues of the rat. In the rat liver, fumarase was predominantly located in the cytosolic and mitochondrial fractions, but not in the peroxisomal fraction. The amount of fumarase associated with the microsomes was less than 5% of the total enzyme activity. The investigation of the intracellular distribution of hepatic fumarase of the rat, mouse, rabbit, dog, chicken, snake, frog, and carp revealed that the amount of the enzyme located in the cytosol was comparable to that in the mitochondria of all these animals. The subcellular distribution of the enzyme in the kidney, brain, heart, and skeletal muscle of rat, and in hepatoma cells (AH-109A) was also investigated. Among these tissues, the brain was the only exception, having no fumarase activity in the cytosolic fraction, and the other tissues showed a bimodal distribution of fumarase in the cytosol and the mitochondria. The mitochondrial fumarase was predominantly located in the matrix. About 10% of the total fumarase was found in the outer and inner membrane, although it was unclear whether this fumarase was originally located in these fractions. No fumarase activity was detected in the intermembranous space.

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