reacts with dioxygen; the second heme is cytochrome a;
the third site, copper-A, is a DINUCLEARcenter.
cytochrome P-450 General term for a group of
HEME-containing MONOOXYGENASEs. Named from the
prominent absorption band of the Fe(II)-carbonyl com-
plex. The heme comprises PROTOPORPHYRIN IX, and
the proximal LIGAND to iron is a cysteine sulfur.
Cytochromes P-450 of microsomes in tissues such as
liver are responsible for METABOLISMof many XENOBI-
OTICs, including drugs. Others, such as the MITOCHON-
DRIAl ENZYMEs from adrenal glands, are involved in
biosynthetic pathways such as those of steroids. The
reaction with dioxygen appears to involve higher oxi-
dation states of iron, such as Fe(IV)O.
cytokine Cytokines are soluble GLYCOPROTEINs
released by cells of the immune system (secreted pri-
marily from leukocytes), which act nonenzymatically
through specific RECEPTORs to regulate IMMUNE
RESPONSEs. Cytokines resemble HORMONEs in that they
act at low concentrations bound with high affinity to a
specific receptor.cytoplasm The part of PROTOPLASMin a CELLout-
side of and surrounding the NUCLEUS. The contents of a
cell other than the nucleus. Cytoplasm consists of a
fluid containing numerous structures, known as
organelles, that carry out essential cell functions.68 cytochrome P-450