Protein deimination in the rat brain: Generation of citrulline-containing proteins in cerebrum perfused with oxygen-deprived media

The central nervous system contains the posttranslational modification enzyme, peptidylarginine deiminase (PAD; EC 3.5.3.15) type II. This enzyme catalyzes the deimination of arginine residues of proteins in a calcium ion-dependent manner, thereby, forming citrulline residues. Previously, we found postmortem deimination of glial fibrillary acidic protein in rat spinal cord. In the present study, therefore, we studied the deimination further as a possible cause of neurodegeneration in cerebrum. Additionally, as a model of tissue damage, the effect of oxygen deprivation on protein deimination in rat cerebral tissue was also analyzed by applying systemic perfusion with an emulsion of perfluorochemicals. In the presence of sufficient oxygen, such perfusion caused only negligible formation of citrullinated proteins. However, deprivation of oxygen produced by the perfusion medium resulted in preferential deimination of many kinds of high-molecular-weight (>55 kDa) proteins. Deiminated protein immunoreactivity was found in all regions of the cerebrum but was relatively more intense in the hypothalamus and the lateral amygdaloid nucleus. Most of the deiminated protein-positive cells seemed to be astrocytes. Some neuronal cells in the lateral amygdaloid nucleus became positive after prolonged perfusion with oxygen-deprived medium, although the PAD immunoreactivity appeared only in astrocytes. These results indicate that PAD type II localizes mainly in astrocytes of the cerebrum and that a consequence of hypoxia is enzyme activation that deiminates proteins in astrocytes and some neurons. Such deimination of proteins may be a useful marker of nerve cell degeneration.