BIOGENESIS OF PEROXISOMES - ISOLATION AND CHARACTERIZATION OF 2 DISTINCT PEROXISOMAL POPULATIONS FROM NORMAL AND REGENERATING RAT-LIVER

According to Poole et al. (1970. J. Cell Biol. 45:408-415), newly synt hesized peroxisomal proteins are incorporated uniformly into peroxisom es (PO) of different size classes, suggesting that rat hepatic PO form a homogeneous population. There is however increasing cytochemical an d biochemical evidence that PO in rat liver are heterogenous, undergoi ng significant modulations in shape and size in process of PO morphoge nesis (Yamamoto and Fahimi, 1987. J. Cell Biol. 105:713-722). In the p resent study, the kinetics of incorporation of newly synthesized prote ins into distinct PO-subpopulations have been studied using short-term in vivo labeling (5-90 min). Two distinct ''heavy'' and ''light'' cru de PO fractions were prepared by differential pelleting from normal an d regenerating liver, and highly purified PO were subsequently isolate d by density-dependent metrizamide gradient centrifugation according t o Volkl and Fahimi (1985. Eur. J. Biochem. 149:257-265). The peroxisom al fractions banded at 1.20 and 1.24 g x cm-3. They differed in their mean diameters and form-factors and particularly in respect to the act ivity of beta-oxidation enzymes which was higher in the ''light'' PO. Whereas the ''light'' PO exhibited a single immunoreactive band with t he antibody to the 70-kD peroxisomal membrane protein the ''heavy'' PO contained an additional (68 kD) band. In pulse-labeling experiments ' 'light'' PO showed clearly a higher initial rate of incorporation than the ''heavy'' PO. The relative specific activity in the ''heavy'' PO fraction, however increased progressively reaching that of ''light'' P O by 90 min. These observations provide evidence for the existence of different PO populations in rat liver which differ in their morphologi cal and biochemical properties as well as in their rates of incorporat ion of new proteins.