Localization of carnitine acetyltransferase activity in brown adipocytes of the rat.

Carnitine acetyltransferase (CAT), an enzyme involved in the beta-oxidation of fatty acids, was localized in the interscapular brown adipose tissue of the rat. The reaction products were localized on the cristae and surface of the inner mitochondrial membrane. Thin needle-like reaction products were abolished in control specimens incubated in either acetyl CoA (substrate)-free medium or inhibitor (HgCl2)-containing medium. They were also abolished when materials were fixed in a mixture of glutaraldehyde (1%) and paraformaldehyde (4%). Even when cristae were disintegrated in certain mitochondria contacting with nearby lipid droplets, the CAT activity was preserved in the rest of the intact cristae. However, when a mitochondrion was completely engulfed in a lipid globule, all cristae lost their CAT activity. These findings suggest an influence of lipid on the mitochondrial CAT activity. In the experimental administration of bezafibrate, a hypolipidemic drug increasing the CAT activity, the CAT activity in the brown adipose tissue was significantly increased (male 6.0-fold, female 2.0-fold) at two weeks of administration of bezafibrate except for castrated male and female rats. However, the localization of CAT activity in mitochondria was not altered by the administration of bezafibrate.