Abstract
ADP-ribosylation reactions are ubiquitous; the enzymes catalyse the transfer of the ADP-ribose portion of NAD to proteins. These reactions seem to regulate a wide variety of cellular functions. ADP-ribosyl transferase (ADPRT) enzymes occur in both the nucleus and the cytoplasm. The nuclear ADPRT synthesises homopolymers of ADP-ribose on suitable acceptor proteins; the polymers may be linear or branched. The nuclear enzyme is confined to nucleated cells, but is found in nucleated protozoa, plants and animals. The cytoplasmic enzymes seem to be able to catalyse only the formation of mono-ADP-ribosyl protein products. These mono-ADPRT enzymes are ubiquitous and occur in bacteria, bacteriophages, protozoa, plants, animals and viruses. The physiological functions of the nuclear enzyme has been shown to include the regulation of DNA repair. It probably also regulates the conformation of chromatin. Nuclear poly(ADP-ribose) consists of a constitutive fraction with a half-life of about 8 hours and an activated fraction, which is produced in response to DNA damage and the consequent DNA strand-breaks; the activated fraction has a very short half-life, possibly less than one minute. The nuclear enzyme may also be involved in certain cases of cell differentiation, in antigenic variation in trypanosomes, in DNA transfection and possibly in malignant transformation.
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