Identification of the Pseudocholinesterase Type in Human Blood Spots

Abstract

This is a test designed to identify different enzymes in blood spots, which may be up to several weeks or months old. The level is estimated by inhibiting the different enzymes by dibucaine and the percentage inhibition is known as the Dibucaine Number. The majority of the population have a DN of 80. 1 in 35 has a DN of 60 and less than 1 in 1,000 has a DN of 20.

Suxamethonium is a muscle relaxant widely used in anaesthesia. Its action is brief because it is quickly destroyed by a substance in the blood called pseudocholinesterase. Some people with prolonged paralysis have been shown to have very small amounts of pseudocholinesterase, and therefore when injected with suxamethonium the action of the latter is prolonged. The condition of low blood pseudocholinesterase can be inherited.

Kalow and Genest (1957)¹ discovered that the pseudocholinesterase deficiency is due to the replacement of the latter by a rare, but similar substance or enzyme of much lower efficiency. These two enzymes may be distinguished by observing their inhibition by a local anaesthetic dibucaine. Experiments show that the usual enzyme is inhibited by about 80 per cent. and the unusual enzyme by about 20 per cent. Mixtures of both enzymes show an intermediate degree of inhibition.

There are also other much rarer variants of the enzyme pseudocholinesterase. The two inherited genes responsible for the presence of pseudocholinesterase are the same in the majority of people, and the majority of people have the usual pseudocholinesterase in the blood. Rather less than 1 in 1,000 people has two unusual genes and these have the unusual pseudocholinesterase and about 1 in 35 has one gene for the usual cholinesterase and one for the unusual cholinesterase.

The measurement of the DN does not depend on the actual level of enzyme concentration, and can be obtained with very small amounts of serum, plasma or whole blood. Experiments have shown that pseudocholinesterase remains stable in the dried state for weeks or even years; e.g., a nine-month-old spot of blood was examined and found to come from a person having one gene for usual and one gene for unusual pseudocholinesterase. A fresh sample of blood was taken from the person from whom the original spot came and the results were confirmed.

It is suggested that the determination of the DN of a human blood spot which can be kept dry is practicable for a long period and may occasionally be found to be of some value in the forensic laboratory.