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A kinetic and thermodynamic study was made of the formation of the hybrid (R(2)Y(2)) glyceraldehyde 3-phosphate dehydrogenase from the yeast (Y(4)) and rabbit (R(4)) enzymes. The values of the thermodynamic parameters for the equilibrium between R(4), Y(4) and R(2)Y(2) suggest that the R(2)-R(2) and Y(2)-Y(2) interactions are similar. However, the failure to observe the RY(3) and R(3)Y hybrids is interpreted in terms of differences at the interfaces of the R-R and Y-Y interactions (the glyceraldehyde 3-phosphate dehydrogenase molecule being regarded as a dimer of dimers). The kinetics of formation of the R(2)Y(2) hybrid were studied and a model was proposed to account for the results. Best-fit values for the rate constants of the individual steps were evaluated by computer simulation, and the rate-limiting steps were identified as the dissociation of tetramers to dimers. It is proposed that the cleavage plane for dissociation of the tetramers corresponds to the region of low electron density through the centre of the molecule in the X-ray-crystallographic structure for human glyceraldehyde 3-phosphate dehydrogenase (Watson et al., 1972), which is probably the plane containing the Q and R axes in the lobster enzyme (Buehner et al., 1974). The R(2)Y(2) hybrid was isolated in milligram amounts by ion-exchange chromatography and its rate of reversion to the native enzyme was shown to be consistent with the kinetic model proposed from the hybrid-formation experiments.

Publisher Summary This chapter focuses on the role of potassium and sodium ions as studied in mammalian brain, and examines the methods of measurement of potassium and sodium ion concentrations in mammalian brain. The mechanism for maintaining the potassium and sodium gradients across the cell membrane yields information about the genesis of the electrical activity. It is highly likely that the transmembrane potassium and sodium gradients themselves are the key “pace setters” of the biochemical mechanisms which create and maintain their gradients at such constant relative levels. Potassium and sodium in the brain also have a clinical importance in relation to the convulsive threshold, to the sodium retention during the depressive phase of manic depression, to the increased frequency of preconvulsive electroencephalographic signs during premenstrual sodium retention, and to the ionic balance following surgery. These ions may determine the rates of the enzymic and nonenzymic reactions which restore them to their own previous dynamic equilibria from which electric or chemical activity has displaced them. The chapter also discusses the methods of study of potassium and sodium in the mammalian brain, maintenance of potassium and sodium gradients in brain cells, and the interrelations of potassium and sodium and the brain's metabolism.

CAVANAGH1, using the technique described by Nakai2, has filmed the growth of isolated cells from chick-embryo dorsal root ganglia. By the application of a similar technique, growth of new processes has been shown in isolated cells from dorsal root ganglia of new-born rats up to 7 days old. The Wag-strain rats were decapitated, the spinal cord separated, and the ganglia teased out with needles under microscopic vision. The ganglia were torn open to release a large number of cells into the culture Medium 199 (ref. 3) in well slides. The slides were covered with cover-slips, sealed with petroleum jelly, and incubated at 37° C, They were examined and photographed under phase-contrast microscopy immediately on incubation (20 preparations), and then three times a week; in this series, cultures from 15 young rats, and 11 adult rats, were observed between 7 and 14 days after the beginning of incubation. No processes were visible in the freshly incubated cells, nor were they in the cells from the adult ganglia (Fig. 1). However, in 13 out of 15 preparations of cells from young rats, straight or angular processes of length equivalent to from one-quarter to two times the cell body diameter could be seen in about a quarter to a third of the cells in each preparation (Fig. 2). Growth cones could sometimes be seen either at the tips of the processes or where they changed direction.