Biomedical subjects
Publications and source records attributed to M Jeyasingham.
New nursing job titles mean little to the general public. Nurses with the same title may vary in knowledge, experience and grade. Introducing a framework for registration of a higher level of practice is raising a number of issues and concerns.
1. The safe limits of alcohol intake are difficult to define because of individual variations in susceptibility to damage. The present recommendations are based largely on epidemiological studies of liver damage. 2. Recent investigations indicate that alcoholic brain damage is much more common than previously suspected. More information is required about its natural history and the characteristics of individuals most likely to suffer damage. 3. Thiamin (vitamin B1) deficiency has long been associated with brain damage and may result from a number of additive causes in the alcoholic patient. New information indicating damage to the protein moeity of some of the thiamin-using enzymes has been reviewed, as have possible mechanisms of brain cell necrosis.
Human red blood cell transketolase has been resolved into two components by gel filtration. One component has its thiamine diphosphate coenzyme firmly bound whilst the other variant of the enzyme is a smaller molecule which is inactive without added thiamine diphosphate, for which it has a reduced affinity. It is concluded that the failure to detect an increase in activation in the commonly used clinical test of red cell transketolase activation by raising the thiamine diphosphate concentration above about 0.3 mmol/l is likely to be due to masking of the effect of activation of the low affinity variant in haemolysates from normal red blood cells by the inhibitory effect of excess thiamine diphosphate upon the activity of the high affinity form of the enzyme with which it is mixed. Increased activation by higher thiamine diphosphate concentrations is sometimes seen in haemolysates from the blood of chronic alcoholics, as well as in the low molecular weight fraction separated from normal haemolysates. It is considered likely that there are at least two variants of the enzyme and that the low molecular weight variant represents a damaged form of the enzyme normally present in small amounts but formed in larger proportions in vivo in abnormal conditions like chronic alcoholism and thiamine deficiency as well as by enzyme breakdown in vitro. In the light of these conclusions some recently proposed hypotheses regarding the role of transketolase in the genesis of brain damage in thiamine deficiency are reconsidered and a modified mechanism is proposed consistent with these and other recent findings.
A modified procedure for the preparation of transketolase from erythrocytes by dialysis and column ion-exchange resin chromatography is described. Changes have been made in the resin used, the column dimensions and the elution procedure so as to separate the enzyme with improved resolution, prepare the apoenzyme free of thiamine pyrophosphate and study the kinetics of its activation or reactivation by the coenzyme. On the basis of the elution profile of the enzyme activity from the chromatographic column, two different samples of the transketolase have been isolated, which differ not only in their isoelectric properties, but also in the proportion of the transketolase present in the apoenzyme form. Not only do the apoenzymes isolated from each of the two fractions differ in the way in which they recombine with thiamine pyrophosphate but kinetic analysis of the results shows that each fraction contains at least two variants of transketolase differing in their affinity for thiamine pyrophosphate. Three, probably four, separate variants have been identified which differ in their affinities for thiamine pyrophosphate over a range greater than 10(4). It is concluded that these two fractions of the enzyme must contain different subsets of the eight isoenzymes of transketolase of differing isoelectric points and that some of these isoenzymes must differ also in their affinity for the coenzyme. The implications of these findings for the Blass and Gibson hypothesis about the pathogenesis of the Wernicke-Korsakoff syndrome are considered.
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