Learning about rules but not about reward is impaired following lesions of the cholinergic projection to the hippocampus.

Common marmosets with bilateral ibotenic acid-induced destruction of the neurones of the vertical limb of the diagonal band of Broca, which provide the major cholinergic input to the hippocampal formation, were impaired on the acquisition but not on the retention of a repeated-trial visuospatial discrimination learning task. They were also impaired on serial spatial reversal learning (but not on serial object reversal learning), on acquisition of a trial-independent successive concurrent discrimination using novel objects (but not on acquisition of a comparable discrimination in which two familiar objects had predictable reward value) and were unable to acquire a difficult conditional object discrimination. It is argued that the role of the hippocampus is in the acquisition but not the retention of ruled-based behaviour (which includes spatial responding) in contrast to the acquisition of discriminations based on stimulus-reward association formation.

Influence of angiotensin II on the concentration of arterial plasma electrolytes in anaesthetized sheep.

Angiotensin II and I significantly raised potassium and lowered sodium and chloride ion concentrations in arterial plasma, with peak changes occurring in the first 2 min of a 6-min infusion period. The octapeptide increased the arterial K+ level in a dose-dependent manner, but the response showed tachyphylaxis when multiple infusions of 6-min duration were administered after a recovery interval of only 5 min. Raising the arterial blood pressure by 20-33 mmHg with adrenaline and noradrenaline failed to account for the increase in arterial plasma K+ concentration produced by the two peptides. These findings, in particular the rise in K+ concentration, are discussed in relation to possible mechanisms by which angiotensin II affects arteriolar tone.

Studies of the regulation and reaction mechanism of the carbamyl phosphate synthetase and aspartate transcarbamylase of bakers' yeast.

Kinetic studies of the carbamyl phosphate synthetase activity (CPSase) of bakers' yeast revealed an absolute requirement for K+ ions ; KM values for two of the substrates, glutamine and bicarbonate, were found to be 5 X 10(-4) M and 3 X 10(-3) M respectively. CPSase activity of the purified enzyme aggregate (M.W. 800,000) was extremely sensitive to UTP with a Ki of 2.4 X 10(-4) M. The purine nucleotide intermediate, XMP, was a strong activator of CPSase, acting at a site different from the regulatory site at which UTP binds ; XMP activation diminished at high concentrations of the substrate Mg-ATP. Studies of the reaction mechanism of CPSase revealed that it involved the sequential addition of the substrates bicarbonate and Mg-ATP, liberation of ADP, addition of glutamine, binding of ATP and then release of ADP and the product carbamyl phosphate. Studies of the reaction mechanism of the aspartate transcarbamylase (ATCase) of the aggregate yielded data which were not compatible with any of the usual models ; whichever reaction mechanism is ultivately found to fit the data, it will probably prove applicable both to the ATCase of the aggregate and to the disaggregated ATCase subunit (MW 138,000).

Kinetics and reaction mechanism of the carbamylphosphate synthetase of a multienzyme aggregate from yeast.

We have studied the kinetics and reaction mechanism of the carbamylphosphate synthetase of an enzyme aggregate functioning in the pyrimidine pathway of yeast. MG--ATP was found to be one of the substrates of the enzyme reaction which was activated by free Mg-2+ and inhibited by free ATP. Feedback inhibition by UTP was non-competitive with respect to both glutamine and bicarbonate. Potassium ions were essential for activity and could not be replaced by sodium. Glutamine could be replaced partially by ammonium ions as nitrogen donor. A bicarbonate-dependent cleavage of ATP was shown to take place in the absence of L-glutamine; L-glutamate was a competitive inhibitor of L-glutamine and the enzyme was shown to synthesize ATP when incubated with ADP and carbamyl phosphate. The reaction mechanism was found to involve sequential addition of the substrates bicarbonate and Mg--ATP and release of ADP, followed by addition of the third substrate glutamine. The purine nucleotide XMP had a pronounced activating effect on the enzyme, acting at a site different from that of UTP. Saturating levels of Mg--ATP eliminated this activation.

Properties of halophil nicotinamide-adenine dinucleotide phosphate-specific isocitrate dehydrogenase. True Michaelis constants, reaction mechanisms and molecular weights.

True values of Michaelis constants of the NADP(+)-specific isocitrate dehydrogenase from Halobacterium salinarium were not very different from those of the apparent constants reported by Aitken et al. (1970). The true constants were affected by salt in a similar manner to that of the apparent constants obtained with NADP(+) at fixed concentrations of 1.0-0.2mm and threo-d(s)-(+)-isocitrate at fixed concentrations of 2.0-0.125mm. The response of apparent V(max.) to salt concentration was highly dependent on fixed substrate concentration in solutions of sodium chloride but much less so in solutions of potassium chloride. At several levels the results emphasize the difficulty of generalizing about the salt relations of a halophil enzyme without adequate attention to substrate concentration. The enzyme has at least two different reaction mechanisms depending on salt concentration. In its ;physiological' form (i.e. in 1.0m-potassium chloride), and also in 1.0m-sodium chloride, the reaction mechanism is ordered with NADP(+) the first substrate added and NADPH the last product released. In 0.25m-sodium chloride, however, the mechanism is different and is probably non-sequential. In 4.0m-sodium chloride with low concentrations of either fixed substrate, there was evidence of a co-operative action of the variable substrate. The evidence suggests that salt participates in the reaction mechanism in two ways: one is the reversible addition to the enzyme in a manner analogous to that of a substrate; the other is dead-end complex-formation. The relative contributions of these two types of reaction determine whether salt activates or inhibits the enzyme. In addition, the inhibition caused by high concentrations of sodium chloride is more complex than the corresponding inhibition by potassium chloride. Gel-filtration experiments indicated that at very low salt concentrations the enzyme has an apparent molecular weight of about 70800. In ;physiological' concentrations of potassium chloride the enzyme appears to be a dimer (mol.wt. 122000-135000) and, in 1.0-4.0m-sodium chloride, it behaves as a trimer or tetramer (mol.wt. 224000-251000). A preliminary method of purifying the enzyme is described.

Properties of a halophil nicotinamide--adenine dinucleotide phosphate-specific isocitrate dehydrogenase. Preliminary studies of the salt relations and kinetics of the crude enzyme.

The effects of chlorides on NADP-specific isocitrate dehydrogenase from Halobacterium salinarium were investigated. The enzyme is stabilized by potassium chloride and sodium chloride and this effect is discussed in relation to the Hill (1913) equation. Kinetics of the enzyme were studied within a range of concentrations of potassium chloride and sodium chloride. Apparent Michaelis constants for both substrates were affected by salt concentration, the effect being greater in sodium chloride than in potassium chloride. Minimal apparent Michaelis constants for both substrates were similar to the corresponding constants reported for yeast isocitrate dehydrogenase. V(max.) was maximal in each salt at a concentration of about 1m. The maximum was higher in sodium chloride than in potassium chloride. At salt concentrations above about 2.3m, the apparent V(max.) was lower in sodium chloride than in potassium chloride, and at salt concentrations below 0.75-1.0m, each salt behaved as a linear activator of the enzyme. Within this concentration range salt and NADP(+) acted competitively; the activation by salt was overcome at finite concentrations of NADP(+). At concentrations above about 1m, potassium chloride was a linear non-competitive inhibitor of the enzyme. Within the range 1.0-2.5m, sodium chloride was also a linear non-competitive inhibitor, but above 2.5m it caused more pronounced inhibition.