CHOLINESTERASE, ACETYLCHOLINESTERASE, AND DIBUCAINE INHIBITION (NUMBER)
There are two isoenzymes of cholinesterase in humans, cholinesterase (also known as pseudocholinesterase) and acetylcholinesterase (or true cholinesterase), both of which catalyze the hydrolysis of acetylcholine to acetate and choline in vitro. However, in the body, only the acetylcholinesterase is involved in the degradation of acetylcholine at the neuromuscular junctions and at synapses. The first of these enzymes, cholinesterase, is secreted by liver cells into the plasma. Cholinesterase is relatively nonspecific in its substrate requirements, and is active in the degradation of many compounds similar to acetylcholine, including some neuromuscular blocking agents. The second enzyme, acetylcholinesterase, is a tissue enzyme found principally in nervous tissue (gray matter), red cells, and muscle. The principal function of acetylcholinesterase in muscle and nervous tissue is to degrade the acetylcholine released at synaptic junctions and into neuromuscular junctions during the firing of appropriate neurons. Very confusingly, the term "cholinesterase", with no descriptors, was also used in some of the older literature to refer to the red cell/muscle enzyme. This use is discouraged to avoid confusion.
The plasma enzyme, cholinesterase, has multiple genetically determined forms with different enzymatic activity profiles. These different activity profiles become clinically significant during administration of anesthesia, when patients with decreased activity may be unable to promptly metabolize some neuromuscular blocking agents. These patients may experience a prolonged (up to several days) paralysis which usually eventually clears without permanent sequelae. These patients recover very slowly from anesthetic agent administration (e.g. succinylcholine) and require prolonged ventilatory support.
The dibucaine inhibition and the number indicating the degree of inhibition is a laboratory measurement of enzyme activity developed to help identify patients with variant cholinesterase activity profiles who may be vulnerable to neuromuscular blockade. The number is determined by measuring the inhibition by dibucaine of cholinesterase enzyme activity. The activity of cholinesterase from normal patients is inhibited 75-83% by the addition of dibucaine. In contrast, patients with the enzyme variant may show either moderately reduced (48-60%) inhibition or markedly reduced (<30%) inhibition of the enzyme. In our laboratory, the ordering of cholinesterase activity automatically triggers determination of the dibucaine number, since the usual reason for ordering cholinesterase studies is to evaluate patients who may have increased susceptibility to prolonged paralysis during anesthesia. Dibucaine inhibition is unnecessary for cases of organophosphate poisoning.
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