Brain guanine deaminase: purification, properties and regional distribution

Abstract

1. 1. Brain guanine deaminase (guanine aminohydrolase, EC 3.5.4.3) has been purified 80-100-fold by a combination of ammonium sulphate fractionation, calcium phosphate gel treatment and DEAE-cellulose fractionation. 2. 2. The rat-brain enzyme showed optimum activity at two values of pH, namely, 6.6 and 9.0 (for sheep brain the corresponding values were pH 6.5 and 8.5); while the liver enzyme manifested a single peak of optimum activity at pH 9.0. 3. 3. The two activities were not resolved during the purification procedure. The rate of denaturation of the two activities by heat was also the same, suggesting the presence of the two activities on the same protein molecule. 4. 4. The Michaelis-Menten constant for guanine was found to be very similar at both pH values: 1.92·10⁻⁵ M at pH 6.6 and 1.98·10⁻⁵ M at pH 9.0. 5. 5. The enzyme had a high specificity of action for guanine, but exhibited no activity on guanosine, guanylic acid, adenine, adenosine and adenylic acid. 6. 6. The enzyme action has been studied on a number of guanine analogues. 2-Aminopurine was not attacked by the enzyme at either pH value. Isoguanine was very slightly deaminated at pH 6.6 and 2-amino-6-mercaptopurine was deaminated to a small extent at both pH values. The results suggest the requirement of oxygen function at position 6 of the purine ring. The replacement of C at position 8 in the purine ring by N further potentiated the enzyme action at pH 6.6. 7. 7. The distribution of the enzyme activity in various parts of the central and peripheral nervous system of monkeys has been studied. The enzyme activity was highest in the thalamus and was fairly abundant in most of the cerebral-cortex regions. No activity could be detected in the cerebellum or the optic nerve. The corpus callosum also had a very low activity.