Mechanisms of Immediate Temperature Compensation: Experiments with Brain Synaptosomes from Rat and Ground Squirrel

Glucose conversion by brain synaptosomes can be regarded as a special case of intrinsic kinetic properties of the enzyme substrate system. Temperature modulation of apparent K(m) for this process can be described with our kinetic model. Using experimental data and the kinetic model, the minimal K(m) value for glucose conversion in ground squirrel synaptosomes was found at the lower temperature (6.5 degrees C), much lower than that for the rat (16.6 degrees C). The inversion temperatures (T(min)) closely coincided with the lowest body temperatures from which the unassisted recovery from hypothermia was demonstrated in both species. This study indicated that thermal modulation of enzyme affinities may have an adaptive role in endotherms that is linked to their tolerance to hypothermia.

Radoslav K. Andjus (1926-2003): a brief summary of his life and work

Radoslav K. Andjus was a professor of physiology and biophysics at the University of Belgrade, Serbia, from 1953 to 1992. He was an elected member of the Serbian Academy of Sciences and Arts, the International Academy of Astronautics, and the Montenegrin Academy of Sciences and Arts. He published over 190 papers in domestic and international journals and three textbooks. The main field of his research was thermophysiology. He studied hypothermia, suspended animation and resuscitation, hibernation and biological rhythms, temperature adaptation and acclimation, and cryoprotection. Professor Andjus also contributed significantly to the fields of brain metabolism, endocrinology, electroretinography, as well as biophysical modeling and theoretical biology.

Kinetic properties of the enzyme-substrate system: a basis for immediate temperature compensation

One-minimum U-shaped temperature profiles of the dissociation constant (K(m)) have been observed experimentally with a variety of enzyme-substrate (E-S) systems. The increase of E-S affinity with falling temperature ("positive thermal modulation of affinity"), which opposes the cold-induced reduction in catalytic velocity, has been often interpreted as significant for both immediate and evolutionary temperature compensations and of major importance in setting thermal limits in ectothermic organisms. This role was denied to enzymes from endotherms, on the ground that their minimal K(m) values were situated well below their normal body temperature. Evidence is presented in this report that affinity changes described by U-shaped profiles can simply be the consequence of intrinsic kinetic properties of the E-S system. Theoretical modeling is achieved by combining the classical expression for the Michaelis constant with Transition State Theory expressions for the three rate constants involved. It provides for the U-shape of the K(m) vs. T profile and allows for the derivation of an equation for identifying its inversion point. Modeling of V(max) and V(min) (reaction velocity under conditions of substrate saturation and of dilution, K(m)>>[S], respectively) is also included. An expression was formulated for predicting the "critical temperature," T(C), corresponding to the low-temperature break in Arrhenius lines. Using existing K(m) data from literature, concerning a variety of E-S systems, our modeling proved to be highly satisfactory. Our own experiments show that glucose uptake by rat brain synaptosomes can be regarded as a special case of basically the same kinetic scheme, and that the U-shaped temperature modulation of apparent K(m) for glucose conversion is also in full agreement with our kinetic modeling. These experiments indicate that positive thermal modulation, although based on intrinsic kinetic properties of the underlying E-S system, may have an adaptive role in endotherms as well, linked, however, to their tolerance to hypothermia.

Effects of xanthine derivatives on electroretinographic responsiveness

In view of the use of synthetic propentofylline (PPF) as a protective agent in brain ischemia, its possible side effects on vision capacities have been explored by electroretinography in comparative experiments with theophylline. We used eyecup preparations of small-spotted dogfish sharks and of European eels, particularly suitable for long-lasting experiments. The drug exerted profound but reversible modifications of ERG records: (1) a dose-dependent increase of the amplitude and duration of the chemically isolated late receptor potential (LRP), (2) a partial unmasking of LRP, (3) a strong potentiation of the LRP-unmasking effect of low temperature, (4) a potentiation of light adaptation effects, and (5) a strong potentiation of the post-illumination hyperexcitability. The effects were explicable as due to a strong phosphodiesterase (PDE) inhibiting, cyclic guanosine monophosphate (cGMP) promoting, action of the drug. The effects were considerably stronger, or even of opposite sign, in comparison to those of the chemically related theophylline. PPF did not seriously affect the ERG c-wave originating in the pigment epithelium. The results suggested that the effects of PPF on vision may not seriously hamper the therapeutic use of the drug. They indicated, on the other hand, that PPF was a retinoactive drug of potential usefulness in the exploration of the complex biochemical events underlying visual transduction.