Neuronal N-acetyl-beta-D-glucosaminidase. Evidence for its biosynthesis in vitro

Vectorial discharge and localization of nascent polypeptides in rough endoplasmic reticulum of developing neuronal perikarya of rat brain cortex

Rough endoplasmic reticulum (RER) prepared from bulk-isolated neuronal perikarya of rat brain cortex of different postnatal ages was found to be active in vectorial discharge of nascent proteins through the membrane; this activity increased with the increasing age of animals and reached maximal values in adults. RER isolated from whole cortical tissue (containing all cell types) exhibited vectorial release only up to 18 days of age; the preparation from adult animals was essentially devoid of secretory activity. Controlled proteolysis of various preparations suggested that in neuronal RER of 8-day-old rats the proportion of nascent proteins operationally retained in the intravesicular space was about twice that retained by cortical preparations. For the purpose of comparison, these parameters were studied also in liver RER.

Distribution and protein synthetic activities of neuronal free and membrane-bound ribosomes during postnatal development of rat cerebral cortex

The distribution and amino acid incorporative activities of free and membrane-bound ribosomes in neuronal perikarya of rat cerebral cortex at successive stages of postnatal development were investigated. The relative proportion of neuronal membrane-bound ribosomes increased significantly between 6 and 18 days of age, reaching 50% of the total ribosomal population around day 18. In contrast to the neuronal fraction, the membrane-bound ribosomes in unfractionated cerebral cortex decreased from 50% at birth to 35% in 18-day-old pups. When tested in a cell-free amino acid incorporation system the activities of both free and membrane-bound ribosomes increased up to day 10 post partum. However, whereas the activity of free ribosomes reached a constant level at this age, that of the membrane-bound fraction continued to rise until 18th day after birth. Crossover experiments employing homologous or heterologous cell sap fractions showed that the preparation from the 18-day-old animals was more efficient in supporting protein synthesis in vitro, if compared with similar preparation from the neonatal animals. This was attributed to an enhanced aminoacylation of transfer RNA in neuronal cell sap of older compared with younger rats. The observed developmental alterations in the distribution and activities of free and membrane-bound ribosomes are discussed in relation to changes that occur in morphology and function of brain during the early postnatal period.

The multiple forms and kinetic properties of the N-acetyl-beta-D-hexosaminidases from colonic tumours and mucosa of rats treated with 1,2-dimethylhydrazine

The separation and purification of the N-acetyl-beta-D-hexosaminidase activities from tumours induced by 1,2-dimethylhydrazine in the rat colon and from colonic mucosa of tumour-bearing animals are reported. Mucosa contained N-acetylhexosaminidases A and B, as well as a third form whose properties with regard to electrophoretic mobility and thermostability lay between those of A and B. Tumours contained only N-acetylhexosaminidase A and B activities. Each form possessed both N-acetylglucosaminidase (EC 3.2.1.30) and N-acetylgalactosaminidase (EC 3.2.1.53) activities, which could not be separated by a variety of techniques. The alteration of the ratio of the two specific activities in each form during purification, together with differences in the kinetic inhibition constants and behaviour during inactivation by various reagents or a temperature of 50 degrees C, supported the belief that each form contains the two enzyme activities, glucosaminidase and galactosaminidase, at separate active sites. This model is in contrast with that reported for these activities from a number of other sources. A variety of treatments reported to cause the conversion of form A into a form resembling B failed to produce such an effect on the rat colonic hexosaminidases.