Recent developments in the synthesis of polysaccharides and their applications are reviewed. Synthetic polysaccharides can be obtained by stepwise coupling, polycondensation, ring-opening polymerization, and orthoester method. Recently, the ring-opeing polymerization of anhydro sugars produced a lot of stereoregular, linear polysaccharides such as 1, 6-glucan (dextran), 1, 6-mannan, 1, 6-galactan, 1, 4-ribopyranan, 5, 6-glucan, and 3, 5-xylofuranan. One of the next targets is to synthesize stereoregular, branched polysaccharides which are thought to have a certain kind of biological functions. Carbohydrate -protein interaction, enzyme action, and immunological function are taken as examples for medical applications of synthetic, linear dextran and mannan.
Hepatic microsomal electron transport system is one of unique models for studying membrane-bound multi-enzyme systems, as the electron transfer between the components involves direct protein-protein interaction in the membrane. Although the determination of the interaction mode between them has been the aim of many studies, no conclusive evidence has been obtained. To elucidate this mechanism, the isolated protein components were incorporated into artificial model membranes and electron transfer was measured directly by stoppedflow techniques. The results obtained indicate the lateral mobility of these protein components in the membrane and the effect of fluidity changes in the membrane suggests that electron transfer between them is a diffusion-limited process. On the contrary, several lines of evidence in favor of heterogeneous distribution of these components in the microsomal membranes has been reported. A mechanism which can explain the discrepancy between model systems and the microsomal system is discussed.
Fibroblastic L cells show spontaneous oscillation (about 4 cycle/min) of membrane potential between -15mV and -40mV under the normal condition, and respond with single hyperpolarizing responses to electrical or mechanical stimuli. In addition, several serum proteins (ex. β-lipoprotein) applied to the membrane induce sustained hyperpolarizing responses. Ionic mechanism of such hyperpolarizing responses is deduced from a variety of elec- trophysiological studies. Mechanical, electrical or chemical stimuli applied to the membrane of L cells elicit Ca2+ influx across the cell membrane. An increase in the intracellular Ca2+ concentration thus induced triggers an openg of so-called Ca2+ -activated K+ channel, and then the K+ conductance increase thus developed produces a hyperpolarizing response. Oscillation would be induced by the periodic changes in the intracellular Ca2+ concentration, which could be brought about by a feed-back relationship between a carrier-mediated Ca2+ entry and an active Ca2+ extruding pump on the plasma membrane. Possible physiological role of a hyperpolarizing response in fibroblasts was also discussed in the light of its ionic mechanism.
The primary response of the typical chemically excitable membranes such as postsynaptic membranes of vertebrate end-plates is the conductance increase of the membrane. The simplest theory which can account for the three elements of a dose-conductance change curve: the maximum response, the affinity of agonist to the receptor and the cooperativity of the curve, is the two-state model. The two-state model is also compatible with the recent findings by the conductance-fluctuation analysis that the unit conductance-increase γ is independent of the kind of agonists on the cholinergic membrane. Various types of the two-state models are formulated based on the generalized twostate model of Kijima & Kijima (1978) which includes both the model of Monod, Wyman and Changeux and that of Koshland. In all types of the two-state model, Hill coefficient at the mid-point of the curve, the measure of the cooperativity, is restricted at small value when the maximum response is small. Some membranes are incompatible with the two-state models due to the above restriction and the three-state model is proposed to account for their responses. The most popular classical model, first proposed by del Castillo and Katz is essentially a kind of multi-state model which assumes the existence of more than two states of receptor-subunits to explain the cooperativity and of another 'active state' of the receptor. This model seems inadequate, however, for the full description of the rate constants of excitation process.
Recent morphological and histochemical studies showed that the neuronal connections in the central nervous system of adult mammals show more flexibility than has been considered before. Partial denervation, for example, often causes sprouting from the remaining intact axons. Our electrophysiological studies on the neurons of red nucleus (RN) of adult cats revealed that elimination of interposito-rubral input which impinge upon somatic portion induces collateral sprouting and synapse-formation of fibers from sensori-motor cortex of the cerebellum. The neurons in the sensori-motor cortex, which normally make connections on the distal dendrite of the RN cell, made new connections on the proximal dendrite after lesion of interpositus nucleus. The newly-formed synapses were functionally effective and had similar properties of synaptic transmission. Collateral sprouting from cortico-rubral fibers also occurs after cross-union of the nerves innervating the forelimb flexor and extensor nerves of adult cats. This sprouting was restricted to the RN neurons innervating the forelimb and was considered to be a consequence of the operation of cross-union. Causes that induces sprouting was discussed in relation to above results and other experiments on neuromuscular synapse-formation.