Subcellular and regional localization ofγ-glutamyl transpeptidase in sheep brain

Abstract

Studies of the subcellular distribution ofγ-glutamyl transpeptidase from sheep brain by discontinuous sucrose density gradient centrifugation showed that 40% of the transpeptidase activity associated with the mitochondrial-synaptosomal fraction was localized with the synaptosomal-enriched fraction. The microsomal fraction was found to have the highest specific activity whenγ-glutamylp-nitroanalide was used as substrate. This activity, however, represented only 5% of the totalγ-glutamyl transpeptidase activity. Approximately 90% of the total enzyme activity was apparently associated with the fraction containing cell debris and membrane fragments.The 160,000g supernatant fluid (soluble supernatant fraction) represented the least total activity, with only 1.2% recovery; however, this fraction contained two apparent forms of the enzyme. One form had a highK mand the other a lowK m for the substrate,γ-glutamylp-nitroanilide.It was observed that the enzymeγ-glutamyl transpeptidase was not evenly distributed in all areas of brain when the homogenate was used as the enzyme source. The brain region with the highest enzyme activity was the thalamus, which was able to form 1.10 μmolp-nitroanaline/min/g wet brain tissue. The cortex was found to have the lowest activity. The 40,000g supernatant fluid from each region, however, exhibited only slight distribution differences.