Library

Notes: In Brain tissue only 10–20 percent of the total hexokinase activity (EC 2.7.1.1) is present in the cytoplasm whereas the particulate activity is associated with mitochondria (Bachelard, 1967; Teich-graber, Biesold and Pigarewa, in press). Either small (Wilson, 1968) or insignificant kinetic differences (Thompson and Bachelard, 1970; Bigl, Müller and Biesold, 1971) between soluble and particulate enzyme activity have recently been reported. Though the DEAE cellulose column chromatography patterns of both hexokinases were reported to be similar (Thompson and Bachelard, 1970; Bigl, Müller and Biesold, 1971), differences were found between soluble and mitochondrial hexokinase after gel electrophoresis separation (Bachelard, 1967; Bigl, Muller and Biesold, 1971). Recently it has been shown in this laboratory that Triton X-100 also enhances the hexokinase activity of the cytoplasm as has already been demonstrated for the mitochondrial form (in preparation).The purpose of the experiments described here was to elucidate whether the soluble cytoplasmic hexokinase differs from the mitochondrial-bound hexokinase in respect to its apparent molecular weight. To diminish the occurrence of experimental artefacts, different extraction procedures were employed. Furthermore, the effect of Triton X-100 on the molecular weight was investigated in an attempt to reveal an explanation for the increase in enzymic activity after incubation of hexokinase with this detergent.

Notes: Abstract— The effects of neonatal thyroidectomy on some constituents of the cerebrum, cerebellum and liver of the rat have been studied during the first 7 weeks of life. In the normal rat between the 6th and 14th post-natal days the RNA content per unit of DNA in the brain increased by 70 per cent. Although the brain continued to grow from the 14th to the 35th day, the amount of RNA relative to DNA decreased by about 20 per cent. The ratio of protein to DNA increased during the whole period studied and in the cerebral cortex it was more than trebled between the age of 6 and 35 days. The growth of the cerebellum extended over a longer period than that of the cerebrum, its weight increasing by 88 per cent between the ages of 14 and 35 days as compared with a cerebral increase of 34 per cent. The DNA content showed a 50 per cent increase during this period.Qualitatively these maturational changes were not affected by neonatal thyroidectomy. Quantitative changes, which applied equally to the cerebral cortex and brain as a whole, were observed. At the age of 35 days, the weights of the cerebral hemispheres and cerebellum were reduced by thyroidectomy by 20 per cent; the overall DNA content per organ did not change, but the amounts of protein and RNA relative to DNA decreased significantly. It is therefore inferred that thyroid deficiency affects the size of the cells in brain and cerebellum rather than their total number. Conversely, the cell population of the liver was only a quarter of that in the control. There was a small but significant decrease in the hepatic protein and RNA content in the hypothyroid animal.The activities of the following enzymes which served as markers for subcellular fractions in homogenates of cerebral cortex were determined: lactate dehydrogenase for the supernatant, glutamate dehydrogenase for the mitochondrial and glutamate decarboxylase for the synaptosomal fractions. When the activities were expressed on a fresh weight basis a significant decrease by comparison with the control values was observed only in the case of glutamate decarboxylase (—15 per cent at the age of 17–32 days); when the activities were based on DNA content all values were reduced, probably as a result of the general decrease in cell size.Pyrimidine metabolism of brain and liver, studied after the administration of [6-14C]-orotic acid, was not affected in either tissue by neonatal thyroidectomy. A small but significant reduction in the incorporation of labelled pyrimidine nucleotides in liver RNA was observed, but no significant decrease in the incorporation in cerebral RNA was found in the hypothyroid rats.