Ultrastructural, immunocytochemical and morphometric characterization of liver peroxisomes in gray mullet, Mugil cephalus

Peroxisomes of the hepatocytes of gray mullets, Mugil cephalus, were charac terized cytochemically and immunocytochemically using antibodies against th e peroxisomal proteins catalase and palmitoyl-coenzyme A (CoA) oxidase. In addition, morphometric parameters of peroxisomes were investigated dependin g on the hepatic zonation, the age of the animals and the sampling season. Mullet liver peroxisomes were reactive for diaminobenzidine, but presented a marked heterogeneity in staining intensity. Most of the peroxisomes were spherical or oval in shape, although irregular forms were also observed. Th eir size was heterogeneous, with profile diameters ranging from 0.2 to 3 mu m. Peroxisomes tended to occur in clusters, usually near the mitochondria and lipid droplets. They also showed a very close topographical relationshi p to the smooth endoplasmic reticulum. Muller liver peroxisomes did not con tain cores or nucleoids as rodent liver peroxisomes, but internal substruct ures were observed in the matrix, consisting of small tubules about 60 nm i n diameter and larger semicircles 120 nm in diameter. The volume density of peroxisomes was higher in periportal hepatocytes of mullets sampled in sum mer than in pericentral hepatocytes, indicating that muller peroxisomes var y depending on physiological and environmental conditions. By immunoblottin g, the mammalian antibodies cross-react with the corresponding proteins in whole homogenates of mullet liver. Paraffin sections immunostained with the antibodies against catalase and palmitoyl-CoA oxidase showed a positive re action corresponding to peroxisomes localized in the hepatocyte cytoplasm. In agreement, the ultrastructural study revealed that catalase and palmitoy l-CoA oxidase are exclusively localized in the peroxisomal matrix in fish h epatocytes, showing a dense gold labeling. The presence of the peroxisomal beta-oxidation enzyme palmitoyl-CoA oxidase in peroxisomes indicated that t hese organelles play a key role in the lipid metabolism of fish liver.