SUMMARY:Mitochondrial and lysosomal preparations from rat livers were examined for the presence of a number of particulate bound enzymes, previously described as being present in mitochondria. The study sought to determine whether these enzymes were truly mitochondrial or partly associated with lysosomes which are contaminants of most mitochondrial preparations. The amino transferases were of particular interest because of their possible involvement in the metabolism of amino acids which are concentrated within lysosomes. It was shown that particulate aspartate amino transferase and alanine amino transferase are truly mitochondrial enzymes. Particulate glutathione reductase was shown to be distributed between lysosomes and mitochondria, the specific activities in the two particles being almost equal. Possible activation of catheptic protein hydrolysis is discussed. Lysosomes were shown not to contain the following enzymes: isocitrate dehydrogenase, glycerol‐3‐phosphate dehydrogenase, serine dehydratase, glucose‐dphosphate dehydrogenase, xanthine oxidase, aldehyde oxidase and fumarate hydratase. Lysosomes were unable to incorporate amino acids into proteins but were able to incorporate acetate14C into fatty acids. The importance of this process as a possible mechanism for controlling or altering the structure of the lysosomal membrane is discussed. Relationships of these findings to some properties of meats are disc
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