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The activities of five peroxisomal enzymes (fatty acyl-CoA oxidase, FAO; uricase, U; L-hydroxyacid oxidase, LHAO; D-aminoacid oxidase, DAO; and catalase, C and the triglyceride content were determined in the regenerating liver of rats after partial hepatectomy. The FAO activity was significantly increased on day 3. Thus, its rise followed the increase of the triglyceride content and preceded the disappearance of steatosis. For U and LHAO a similar progressive increase in activities was obtained on days 3, 5, and 7 of regeneration. DAO exhibited a highly significant increase on day 1. C increased on day 1 and then again after day 3. It is concluded that peroxisomal functions are not restored at parallel rates during liver regeneration.

The variations in liver peroxisomes and in peroxisomal enzymes were studied in the rat during adaptation to cold (5 degrees C). An increase in the number and in the volume and surface fractions of peroxisomes was detected by day 7. Qualitative ultrastructural changes of the peroxisome compartment were observed. Several peroxisomal enzyme activities were found to exhibit a significant increase with different temporal patterns. These results are discussed with regard to the possible contribution of liver peroxisomes to non-shivering thermogenesis.

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