The Journal of Cell Biology, Vol 91, 212-220, Copyright © 1981 by The Rockefeller University Press
Distribution and induction of cytochrome P-450 in rat liver nuclear envelope
S Matsuura, R Masuda, K Omori, M Negishi and Y Tashiro
Induction of cytochrome P-450s by 3-methylcholanthrene (MC) and
phenobarbital (PB) and distribution of P-450s in the rat liver nuclear
envelope were investigated by biochemical analyses and ferritin
immunoelectron microscopy using specific antibodies against the major
molecular species of MC- and PB-induced cytochrome P-450. It was found, in
agreement with Kasper (J. Biol. Chem., 1971, 246: 577-581), that the total
amount of cytochrome P-450s determined by biochemical analysis was markedly
increased by MC, but not by PB, treatment. Immunoelectron microscopic
analysis, however, showed marked and slight increases in ferritin labeling
by MC and PB treatment, respectively. The latter finding was interpreted as
resulting from the induction of a particular molecular species of
PB-induced cytochrome P-450s. Ferritin immunoelectron microscopic analysis
of intact isolated nuclei, naked nuclei from which the outer membrane of
the nuclear envelope was partially detached (mechanically), and isolated
nuclear envelopes have shown that the ferritin particles are found
exclusively on the cytoplasmic face of the outer nuclear envelopes. Neither
the nucleoplasmic face of the inner membrane of the nuclear envelope nor
the cisternal face of both membranes of the nuclear envelope showed any
labeling with ferritin. This indicates that cytochrome P-450 is located
only on the outer membrane of the nuclear envelope and does not diffuse
laterally into the domain of the inner membrane of the nuclear envelope
across the nuclear pores. Our results suggest that a marked heterogeneity
exists in the enzyme distribution between the outer and inner membrane of
the nuclear envelope and that microsomal marker enzymes such as cytochrome
P-450 exist exclusively in the outer membrane. In addition, it appears that
cytochrome P-450 is probably not a transmembrane protein but an intrinsic
protein located on the cytoplasmic face of the outer membrane of the
nuclear envelope.