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Isoforms of alkaline phosphatase determined by isoelectric focusing in patients with chronic liver disorders

Fractionation of alkaline phosphatase isoenzymes and isoforms by isoelectric focusing is a simple procedure that resolves up to 17 fractions having alkaline phosphatase activity. The fractions are stable at 4 degrees C, and undergo only slight changes during repeated freeze-thaw cycles. Pretreatment with phospholipase-C or sialidase changes the isoelectric focusing patterns of alkaline phosphatase in serum; we recommend they not be used owing to the loss of information. We found that the alkaline phosphatase fractions provide diagnostic information in addition to that given by the common liver-function tests in patients with chronic liver diseases. Primary biliary cirrhosis and primary sclerosing cholangitis showed similar biochemical changes, but they are very different from alcoholic cirrhosis based on the common liver-function tests and the alkaline phosphatase isoform patterns obtained by isoelectric focusing. Analysis of the laboratory data using neural networks has some limited use in distinguishing chronic and chronic-active hepatitis of any cause. We have confirmed the tissue assignments made by Griffiths (Prog Clin Biochem 1989; 8:63-74) for the alkaline phosphatase fractions in liver as obtained by isoelectric focusing: Fractions 1a and 1b show a strong correlation with biliary diseases, and fractions 2, 3, and 4 show consistent increases in patients with primary disorders of hepatocytes; these fractions have good sensitivity for hepatocyte injury, but their specificity is limited. Fraction 10 may be a marker of activated T-lymphocytes; it was abnormal in most of our patients suggesting that it is a sensitive but non-specific test. Analysis of alkaline phosphatase by isoelectric focusing deserves further evaluation, because it may facilitate the diagnosis of certain chronic liver disorders and could be a supplement to the biopsy.

Interpretation and clinical significance of alkaline phosphatase isoenzyme patterns

Alkaline phosphatase (ALP, EC 3.1.3.1) is a membrane-bound metalloenzyme that consists of a group of true isoenzymes, all glycoproteins, encoded for by at least four different gene loci: tissue-nonspecific, intestinal, placental, and germ-cell ALP. Through posttranslational modifications of the tissue-nonspecific gene, for example, through differences in carbohydrate composition, bone and liver ALP are formed. Nowadays, most commercially available methods for separating or measuring ALP isoenzymes are easy to perform and sensitive and allow for reproducible and quantitative results. As more isoenzymes and isoforms have been characterized, confusion has arisen due to the many different names they were given. For the sake of simplicity and because of structural analogies, we propose an alternative nomenclature for the ALP isoenzymes and isoforms based on their structural characteristics: soluble, dimeric (Sol), anchor-bearing (Anch), and membrane-bound (Mem) liver, bone, intestinal, and placental ALP. Together with lipoprotein-bound liver ALP and immunoglobulin-bound ALP, these names largely fit the many forms of ALP one can encounter in human serum and tissues. The clinically relevant isoenzymes are sol-liver, Mem-liver, lipoprotein-bound liver, and Sol-intestinal ALP in liver diseases, and Sol-bone and Anch-bone ALP in bone diseases. Many different isoenzyme patterns can be found in malignancies and renal diseases. This test provides the clinician with valuable information for diagnostic purposes as well as for follow-up of patients and monitoring of treatment. However, ALP isoenzyme determination will only provide clinically useful information if the patterns are correctly interpreted. In this respect, care should be taken to use the proper reference ranges, taking into account the age and sex of the patient. A normal total ALP activity does not rule out the presence of an abnormal isoenzyme pattern, particularly in children. Separating ALP into its isoenzymes adds considerable value to the mere assay of total ALP activity.