Biochemistry, Pseudocholinesterase

Fundamentals

Pseudocholinesterase functions primarily as a serine hydrolase that catalyzes the hydrolysis of choline and noncholine esters and maintains active aryl acrylamide. Therefore, pseudocholinesterase amplifies the activity of proteases, such as trypsin, in the body. Pseudocholinesterase predominantly influences neuronal activity, particularly within the hippocampus, amygdala, thalamus, and deep layers of the cerebral cortex.[7] Notably, pseudocholinesterase seems to contribute to the maturation of the central and peripheral nervous systems by regulating neuronal growth and cellular proliferation, specifically at the onset of differentiation and in the early stages of neuronal development.[8]

Issues of Concern

Deficient pseudocholinesterase activity impairs the hydrolysis of choline ester neuromuscular blocking agents, including succinylcholine and mivacurium, commonly administered during endotracheal intubation and surgical procedures. Consequently, these agents persist at the NMJ, resulting in sustained activation of nicotinic acetylcholine receptors and prolonged neuromuscular blockade.

Pseudocholinesterase dysfunction occurs through two primary mechanisms: an acquired reduction in plasma enzyme levels or the inheritance of genetic variants with decreased catalytic activity against choline esters. Additionally, clinical response to neuromuscular blocking agents correlates with the severity of pseudocholinesterase deficiency, with affected individuals experiencing prolonged apnea and paralysis of variable severity.

In pseudocholinesterase deficiency, the expected duration of succinylcholine-induced paralysis is prolonged from approximately 4 to 6 minutes to as long as 8 hours. Increased sensitivity is also observed with exposure to organophosphate agricultural pesticides and ester local anesthetics, including cocaine and procaine.[9]

Prolonged ventilatory support is required in persistent respiratory depression resulting from succinylcholine administration in the setting of pseudocholinesterase deficiency. Positive-pressure ventilation should be continued until skeletal muscle regains adequate neuromuscular function following passive diffusion of succinylcholine from the NMJ.[10] Known pseudocholinesterase deficiency status should be disclosed to all medical personnel through a medical alert band, notifying clinicians of increased sensitivity to mivacurium and succinylcholine in the event of an emergency.

Molecular Level

The pseudocholinesterase enzyme is encoded by the BCHE gene on chromosome 3 (3q26.1–q26.2). BCHE contains 4 exons that are processed to yield a mature protein of 574 amino acids.[11]

The BCHE gene exhibits extensive genetic polymorphism, resulting in marked interindividual variability in pseudocholinesterase activity. Identified variants include qualitative defects that alter enzyme function and quantitative defects that reduce enzyme abundance, including silent alleles associated with minimal or absent enzymatic activity.[12] More than 75 BCHE variants have been identified, with mutations affecting enzyme structure, catalytic efficiency, protein stability, or gene expression. These alterations may result in complete loss of pseudocholinesterase synthesis in severe cases.

Point mutations result in altered enzyme structure secondary to changes in mRNA and translated amino acid sequences, often producing abnormal enzymatic function (qualitative mutations). Mutations leading to stop codons or frameshifts typically result in the synthesis of dysfunctional enzymes (quantitative mutations).

Multiple molecular forms of pseudocholinesterase exist, with subvariants defined by the number and orientation of subunits, ranging from monomers (G1) to symmetric dimers (G2) and disulfide-linked tetramers (G4).[13] Newly translated pseudocholinesterase molecules are released into plasma as monomers. Predominant functional circulation occurs as a glycosylated soluble tetramer, which demonstrates the greatest stability and half-life.

Testing

Pseudocholinesterase deficiency can be identified based on family history or a clinical history of prolonged apnea and paralysis following administration of a depolarizing neuromuscular blockade with succinylcholine. The condition follows an autosomal recessive inheritance pattern, with heterozygous carriers substantially more common than homozygous affected individuals. Family members of known homozygotes should be considered for testing for BCHE gene variants.[14] Confirmation of pseudocholinesterase deficiency can be achieved by sequencing the BCHE gene at 3q26.1. Additional diagnostic approaches include deletion and duplication analysis, targeted variant analysis, BCHE sequencing, and broader genomic sequencing when indicated. The resulting genotypes are compared with known mutations to assess the phenotypic risk of prolonged apneic episodes following exposure to choline esters.

Dibucaine number determination may also be used as a screening test to assess functional pseudocholinesterase activity and identify likely abnormal variants before genetic testing. Dibucaine is an amide local anesthetic that inhibits approximately 80% of normal pseudocholinesterase activity. In contrast, atypical pseudocholinesterase variants exhibit reduced sensitivity to dibucaine inhibition, with homozygous atypical enzymes demonstrating inhibition as low as 20% to 30%. A result greater than or equal to 70% is considered within normal limits, consistent with a functional homozygous typical genotype. A dibucaine number of 20% to 30% inhibition suggests a homozygous atypical pseudocholinesterase variant associated with significantly prolonged neuromuscular blockade in response to choline esters. Heterozygous individuals typically demonstrate dibucaine numbers between 50% and 70%.

Dibucaine number results can assist in clinical planning when genetic confirmation has not yet been completed. Genetic testing may be considered in individuals with abnormal dibucaine number results to confirm the diagnosis and characterize the underlying BCHE variant. Healthcare providers should recognize that the dibucaine number reflects enzyme inhibition phenotype and does not directly quantify serum pseudocholinesterase concentration. A dibucaine number greater than 70% does not preclude quantitative variants associated with clinically significant decreases in plasma enzyme levels.

Pathophysiology

Pseudocholinesterase deficiency can be inherited or acquired. The inherited form is typically transmitted in an autosomal recessive pattern, with heterozygous and homozygous variants occurring in approximately 1 in 25 to 50 and 1 in 2,000 to 5,000 individuals, respectively. The prevalence of hereditary pseudocholinesterase deficiency varies among populations and has been reported more frequently in individuals of European ancestry than in Asian populations.

Acquired reductions in pseudocholinesterase activity may result from a variety of physiologic and pathologic conditions, including malnutrition, pregnancy, extensive burns, malignancy, liver disease, and renal disease. In contrast, obesity and chronic alcohol use have been associated with increased pseudocholinesterase activity. Numerous medications, including aspirin, metoclopramide, monoamine oxidase inhibitors, oral contraceptives, and anticholinesterase agents, can contribute to enzyme dysfunction.

The broad substrate specificity of pseudocholinesterase enables the hydrolysis and sequestration of diverse endogenous and exogenous compounds, including succinylcholine, mivacurium, cocaine, and certain organophosphate-containing agents.[15] Although pseudocholinesterase deficiency is typically asymptomatic at baseline conditions, clinically significant manifestations may occur following exposure to susceptible substrates, particularly when enzyme activity is markedly reduced.

Clinical Significance

Evidence regarding the role of pseudocholinesterase deficiency in the progression of Alzheimer's disease remains poorly characterized. The cholinergic hypothesis of Alzheimer's disease pathogenesis suggests that loss of cholinergic basal forebrain neurons and a relative deficiency of choline acetyltransferase in Alzheimer's disease brains contribute to impaired cholinergic neurotransmission by reducing acetylcholine levels.[16]

Acetylcholine is primarily inactivated by AChE, with pseudocholinesterase contributing to a lesser extent. Increased pseudocholinesterase levels and altered enzyme structure or function have been observed in Alzheimer's disease, particularly within amyloid plaques and neurofibrillary tangles. Pseudocholinesterase may also demonstrate a synergistic role with ApoE4 (apolipoprotein E ε4) in the development of mild cognitive impairment.[17]

Current medications approved by the US Food and Drug Administration for Alzheimer's disease and Parkinson's disease include donepezil and rivastigmine, both cholinesterase inhibitors that increase acetylcholine availability at neuronal synapses.[18] Donepezil functions as a selective AChE inhibitor, whereas rivastigmine acts as a dual AChE–pseudocholinesterase inhibitor.

Rivastigmine, through dual inhibition of AChE and pseudocholinesterase, has been associated in recent studies with potential advantages in cognitive and executive functional outcomes compared with donepezil.[19] Current studies are evaluating selective pseudocholinesterase inhibitors as potential therapeutic agents for Alzheimer's disease in preclinical settings.[20][21]

Pseudocholinesterase is known to metabolize cocaine. Therefore, genetic variants that alter catalytic activity may prolong euphoric effects. Such findings suggest that pseudocholinesterase variants can serve as genetic markers for susceptibility to drug dependence and addiction.[22] Plasma cholinesterase levels may be significantly elevated in individuals with substance use disorders, such as cocaine use disorder, suggesting a potential role in addiction pathophysiology.[23] Furthermore, BCHE gene amplification and elevated enzyme activity have also been observed in tumorigenesis and neurologic disorders, potentially providing opportunities to target gene expression as a therapeutic modality.

Organophosphates are irreversible cholinesterase inhibitors found in chemical warfare agents and pesticides. Serum cholinesterase hydrolyzes and inactivates organophosphates, playing an important role in limiting systemic toxicity from these compounds. Consequently, pseudocholinesterase levels decrease in proportion to the severity of organophosphate poisoning and may provide an early indication of toxicity before the onset of symptoms. Measurement of AChE and pseudocholinesterase levels and activity in blood samples serves as a rapid, reliable, and low-cost screening tool in field settings with limited resources. Serial measurements may also be useful for monitoring the severity of toxicity and treatment response.[24]

Hepatocytes function as the primary site of plasma cholinesterase synthesis. Serum plasma cholinesterase levels may serve as a valuable biomarker of liver function, demonstrating strong correlation with established indicators, such as serum albumin, prothrombin time/international normalized ratio, and the Model for End-Stage Liver Disease (MELD) score. Serum cholinesterase levels may also serve as a prognostic marker in advanced liver disease, distinguishing between decompensated cirrhosis, characterized by lower enzyme levels, and compensated cirrhosis, marked by higher enzyme levels.[25]

Pseudocholinesterase has also been proposed as a potential biomarker for Wilson disease, a rare disorder characterized by excessive copper accumulation in the liver and brain due to impaired metabolism, which can lead to hepatic cirrhosis and neurologic decline.[26] A 2022 case report indicates that pseudocholinesterase deficiency should be excluded prior to using pseudocholinesterase levels as a diagnostic or prognostic marker for inherited disorders such as Wilson's disease. Diagnostic limitations occur when pseudocholinesterase deficiency and disease-related alterations coexist, rendering pseudocholinesterase levels unreliable as a marker of disease severity. Patients with Wilson disease receiving copper chelation therapy who demonstrate improvement in other liver biomarkers despite persistently reduced pseudocholinesterase levels should be evaluated for underlying genetic pseudocholinesterase deficiency.[27]

In addition to its association with hepatic status, pseudocholinesterase activity may also correlate significantly with red blood cell and renal function.[28] Although pseudocholinesterase is absent in red blood cells, serum levels are directly proportional to hemoglobin and hematocrit. Serum cholinesterase concentrations have also been shown to be directly proportional to blood urea nitrogen and creatinine, which are established indicators of renal function.