Biomedical subjects
Publications and source records attributed to N Lotan.
Enzyme-based artificial organs are being developed as metabolic assist devices. These are required when normal metabolism is impaired, or when the body is overloaded by undesired metabolites or toxins. The implementations of this approach for treating a genetic disease, and for metabolic support in liver failure are envisaged. The kinetic aspects and mass transfer characteristics of bioreactors for these systems are considered in detail.
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In a previous report, we presented a new analytical model describing the performance of a packed-bed catalytic unit, where the reaction between two cosubstrates is catalyzed by an enzyme immobilized on a porous carrier. The model explicitly takes into account the changes in concentrations of both cosubstrates along the reactor, as well as the hydrodynamic regimen (i.e., back-mixing) prevailing in the packed bed. In the present report, and on the basis of the procedures developed, we present a detailed analysis of the performance of the reactor. With numerical simulations, the effects of internal diffusion limitations, the depth of the pores, the substrates' concentration in the feed, and kinetic parameters are evaluated. Particular attention is also given here to the back-mixing effects prevailing in the reactor. An experimental procedure for assessing their extent is described.
Familial hypercholesterolaemia is caused by genetic defects in the cellular metabolism of cholesterol (C) and is characterized by high levels of low-density lipoproteins (LDL) and premature atherosclerosis. The C is carried in the plasma mainly as an LDL-C complex, and removal of the latter from plasma is highly desirable. This task can be achieved by selective haemoperfusion (HP), thereby eliminating the need for plasmapheresis. Agarose beads (2 per cent agarose, 0.85 to 1.4 mm in diameter) were prepared, and crosslinked with epichlorohydrin. Heparin and/or ethanolamine were subsequently attached. The beads thus obtained were found to be suitable for the removal of LDL-C from the whole blood of hypercholesterolaemic rabbits, using a simple HP technique. A single two-hour HP treatment with a 40 ml column packed with active agarose beads resulted in a 30 per cent decrease in the C plasma level in the experimental animals. Our previous, in vitro, studies with the plasma of hypercholesterolaemic patients showed a high selectivity of the beads for LDL. Yet when used with hypercholesterolaemic rabbits, a relatively high amount of HDL was also removed from the blood. This can be attributed to a significant difference between the structure of human hypercholesterolaemic lipoproteins and that of rabbits. Upon treatment of blood using the active agarose beads, no abnormalities in plasma and blood composition were detected, except for some prolongation of PT. It is to be hoped that this new system will replace the presently used and highly expensive plasmapheresis.
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The degradation pathway of a synthetic biopolymer, poly-N5-(2-hydroxyethyl)-L-glutamine, by papain under physiological conditions, was extensively investigated. The enzymic reaction was found to be rather complex: it progressively slows down, and comes to an end when the degradation fragments are tetrapeptides. In order to account for this phenomenon, a modified Michaelis-Menten kinetic model is proposed, in which Km is assumed to be dependent on the degree of polymerization of the macromolecular substrate, and therefore varies as the degradation reaction proceeds. The new model accurately describes all the experimental data, and allows one to predict the entire course of the reaction. The behaviour of the system considered is interpreted to represent a model of biological recognition at the molecular level.