B-type cytochromes in plasma membranes isolated from rat liver, in comparison with those of endomembranes.

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Abstract
Fractions of plasma membranes, Golgi apparatus, endoplasmic reticulum (ER) and nuclear envelope were isolated from rat liver and were characterized by EM and biochemical methods. The purity of the fractions was controlled by morphometry and by marker enzyme activities. Amounts of cytochromes b5, P-450, and P-420 and the NADH- and NADPH-cytochrome c reductase activities were measured. The pigments of the microsomal electron transport system were found in all membrane fractions in relatively high amounts, thus excluding an origin by microsomal contamination. Purified preparations of plasma membrane and Golgi apparatus contained .apprx. 30% of the cytochromes P-450 + P-420 found in ER membranes. Plasma membranes were also characterized by a high ratio of P-420/450. Degradation of cytochromes P-450 and P-420 was relatively rapid in all fractions, except in the ER. Cytochrome b5 extracted from plasma membranes was spectrophotometrically and enzymatically indistinguishable from ER cytochrome b5. However, immunological characterization with rabbit antibodies against the trypsin-resistant core of microsomal cytochrome b5 showed the presence of at least 2 types of cytochrome b5 in ER membranes, in contrast to the plasma membranes in which only one of these components was detected. This immunological differentiation also demonstrates that the plasma membrane-bound cytochrome b5 is endogenous to this membrane and does not reflect contamination of ER elements. Cytochromes b5, P-450 and P-420 are apparently not confined only to ER and nuclear membranes but also occur in significant amounts in Golgi apparatus and plasma membranes. The findings are discussed in relation to observations of similar redox components in Golgi apparatus, secretory vesicles and plasma membranes of other cells.