Macromolecular aspartate aminotransferase

Distribution of serum concentrations reported for macroenzyme aspartate aminotransferase (macro-AST)

Background

The presence of macroenzyme (M) is often the explanation of an isolated elevation of aspartate aminotransferase (AST). Where M is identified, it is reasonable for the clinician to ask where an individual patient's result fits in with known concentrations of M. In this context, we conducted a survey of literature to examine the distribution of reported serum concentrations of macro-AST. We also analyzed the distribution data to examine whether elevations were consistent with simple alteration of circulatory half-life (t_1/2) of M relative to normal AST.

Methods

Distributions of M were compiled from the literature. These distributions were compared to predictions based on fixed changes in t_1/2 applied to the reference interval for AST.

Results

There was a bimodal distribution of literature values for M (n =51), comprised roughly of populations A (M <200 U/L; 60% of total) and B (M >200 U/L; 40% of total). The two distributions were reasonably well characterized by a simple projection to the right of the reference interval for AST according to increased t_1/2 (A: t_1/2 =3.3 days; B: t_1/2 =19.8 days) relative to AST (t_1/2 =0.7 days).

Conclusions

Knowledge of distributions for M may be useful in discussion with clinicians regarding significance of M for individual patients. Distributions for M were consistent with the simplest explanation for elevated AST due strictly to an extended circulatory lifetime for M. Caveats to analysis, however, include selection within literature data mainly for patients with various co-morbidities.

[Macro-AST and myeloma: An incidental association?]

INTRODUCTION

Macro-AST is recognized as a classical aetiology of isolated and persistent increase of serum aspartate aminotransferase (AST) levels. Macro-AST are high molecular weight complexes associating AST and a macromolecule, often an immunoglobulin. Although those macroenzymes of unknown pathogenesis are usually non-pathogenic, association with several diseases, including autoimmune diseases and liver diseases has been described.

CASE REPORT

We report here the case of a 45-year-old patient with previously normal liver enzymes in whom an AST elevation and an IgA monoclonal gammopathy were discovered concomitantly. Following the diagnosis of multiple myeloma, we could evidence in the patient's serum a complex between AST and the monoclonal IgA. AST levels course followed closely the progression of monoclonal gammopathy.

CONCLUSION

This is the first report demonstrating a clear link between macro-AST and a monoclonal gammopathy.

Macro-aspartate aminotransferase (macro-AST). A 12-year follow-up study in a young female

We describe a case of chronic elevation of serum aspartate aminotransferase (AST) activity due to the presence of a macro-enzyme form of AST (macro-AST) in a young female followed up for 12 years. This biochemical abnormality, although generally detected in acute and chronic hepatitis, malignancies and autoimmune disease, was not associated with any particular illness and did not seem to be congenital, as normal AST concentrations were reported in the patient's family members. Moreover, the normal or quasi-normal values of AST associated with peak levels observed in our case suggest that this phenomenon has a fluctuating behaviour rather than persisting for many months or years. In conclusion, we believe that it is important: 1) to reassure the patient with macro-AST, as this condition has a benign evolution and does not require any specific treatment; 2) to keep in mind this biochemical abnormality in all cases of unexplained AST elevation in order to avoid unnecessary invasive diagnostic procedures.

Does breast feeding influence liver biochemistry?

OBJECTIVE

It is assumed that early feeding can affect liver biochemistry because breast-fed infants have a higher risk of hyperbilirubinemia than formula-fed infants. The authors sought to determine how feeding mode affected liver biochemistry in healthy term infants.

METHODS

Healthy term infants were followed up during infancy with a monthly questionnaire about feeding mode. Blood samples were obtained at 2, 6, and 9 months. Liver biochemistry (serum albumin, alkaline phosphatase, lactic dehydrogenase, aspartate aminotransferase [AST], and bilirubin), total insulin-like growth factor 1 (IGF-I), and insulin growth factor binding protein 3 (IGFBP-3) were determined at all ages.

RESULTS

Mean AST and bilirubin were significantly higher in breast-fed infants at 2 and 6 months. In addition, mean albumin levels were higher in breast-fed infants at 2 months. Alkaline phosphatase, IGF-I, IGFBP-3, and lactic dehydrogenase levels did not differ between the feeding groups. AST levels did not correlate significantly with bilirubin, albumin, alkaline phosphatase, or lactic dehydrogenase values. There was a strong positive association between AST and IGF-I at 2 months (r = 0.47, P = 0.004).

CONCLUSION

Cytomegalovirus infection, vitamin K deficiency, and macromolecular forms of AST could be an explanation for a higher AST level among breast-fed infants. However, no other clinical or paraclinical sign of liver disease was seen, all infants were given oral vitamin K, and the AST did not rise to levels comparable to those seen in individuals with macromolecular AST. The authors speculate the most likely explanation of the elevated AST is induction of hepatocytes by factors in human milk. This is supported by the higher albumin levels in breast-fed infants and the positive association between AST and IGF-I.

Macro-aspartate aminotransferase in a female with antibodies to hepatitis C virus

Persistent elevation of aspartate aminotransferase (AST) activity in serum due to the presence of a macroenzyme form of AST (macro-AST) may lead to diagnostic confusion in many clinical conditions, particularly those associated with chronic liver disease. We describe a case of macro-AST arising in an adult female with a false-positive hepatitis C virus (HCV) RNA test result that was not accompanied by other biochemical or histologic evidence of liver disease. The presence of macro-AST in serum was confirmed utilizing size-exclusion, high performance liquid chromatography (HPLC) and Protein G-agarose beads to precipitate immune complexes of AST and immunoglobulin G followed by centrifugation and AST activity measurements in the supernatant. A brief review of the clinical enzymology of AST and methods used to quantify serum macro-AST activity is provided.