Hepatic alkaline phosphatase isoenzymes: isolation, characterization and differential alteration

ALP-activated probe for diagnosis of liver injury by multispectral optoacoustic tomography

In this chapter, we highlight the advantages of multispectral optoacoustic tomography (MSOT) technique and the activatable photoacoustic probes in the biomedical field, and give a brief introduction to enzyme-activated probes for disease diagnosis and therapeutic outcome evaluation. We also present a detailed description of the procedures for the synthesis of an activatable small molecule probe C₁X-OR₁ and confirmation of its specific response to alkaline phosphatase in solution and cells. With MSOT, the liposomal C₁X-OR₁ can be utilized for detection of hepatic ALP as well as for in vivo diagnosis of drug-induced liver injury in a three-dimensional manner.

Activatable probes for diagnosing and positioning liver injury and metastatic tumors by multispectral optoacoustic tomography

Optoacoustic tomography (photoacoustic tomography) is an emerging imaging technology displaying great potential for medical diagnosis and preclinical research. Rationally designing activatable optoacoustic probes capable of diagnosing diseases and locating their foci can bring into full play the role of optoacoustic tomography (OAT) as a promising noninvasive imaging modality. Here we report two xanthene-based optoacoustic probes (C1X-OR1 and C2X-OR2) for temporospatial imaging of hepatic alkaline phosphatase (or β-galactosidase) for evaluating and locating drug-induced liver injury (or metastatic tumor). The probes rapidly respond to the disease-specific biomarkers by displaying red-shifted NIR absorption bands and generate prominent optoacoustic signals. Using multispectral optoacoustic tomography (MSOT), we can precisely localize the focus of drug-induced liver injury in mice using C1X-OR1, and the metastatic tumors using C2X-OR2. This work suggests that the activatable optoacoustic chromophores may potentially be applied for diagnosing and localizing disease foci, especially smaller and deeper ones.

Liver Histopathology and Liver and Serum Alanine Aminotransferase and Alkaline Phosphatase Activities in Epileptic Dogs Receiving Phenobarbital

Phenobarbital (PB) therapy is frequently associated with elevated serum alanine aminotransferase (ALT) and alkaline phosphatase (AP) activities in dogs without clinical signs of liver disease. The goal of this study was to determine if increased serum ALT and AP activities in clinically healthy PB-treated epileptic dogs are due to hepatic enzyme induction or to subclinical liver injury. Liver biopsies were obtained from 12 PB-treated dogs without clinical signs of liver disease but with elevated serum ALT and/or AP activities or both. Liver biopsies were obtained from eight healthy control dogs not receiving PB. Biopsies were evaluated histopathologically (all dogs) and liver homogenates were assayed for ALT (all dogs) and AP (six treated dogs, all controls) activities. As a positive control, liver cytochrome P4502B, an enzyme known to be induced by PB, was measured by benzyloxyresorufin- O-dealkylase activity and immunoblotting (five treated dogs, all controls). Serum AP isoenzyme analyses were performed. Results showed that ALT and AP activities in liver homogenates were not increased in treated dogs compared with controls, whereas the positive control for induction, CYP2B, was dramatically increased in treated dogs. Histopathological examination of liver biopsies revealed more severe and frequent abnormalities in treated dogs compared to controls, but similar types of abnormalities were found in both groups. Serum AP isoenzyme analyses in treated dogs demonstrated increased corticosteroid-induced and liver isoenzyme activities compared to controls. Results do not support induction of ALT or AP in the liver as the cause of elevated serum activities of these enzymes due to PB.

Serum alkaline phosphatase isoenzyme profiles in phenobarbital-treated epileptic dogs

BACKGROUND

Serum total alkaline phosphatase (AP) activity commonly is high in dogs receiving phenobarbital. Specific isoenzymes responsible for this increase are not well documented.

OBJECTIVES

The purposes of this study were 1) to qualitatively and quantitatively describe serum AP isoenzymes in phenobarbital-treated dogs and 2) to monitor changes in serum AP isoenzyme activities associated with phenobarbital treatment over time.

METHODS

Serum AP isoenzyme activities were determined in a cross-sectional study of 29 dogs receiving phenobarbital (duration of treatment 2 months to 6.5 years). Additionally, in a prospective study of 23 dogs, serum AP isoenzyme activities were determined before and 3 weeks, 6 months, and 12 months after the start of phenobarbital treatment. Isoenzyme activities were quantitatively determined using wheat germ lectin precipitation and levamisole inhibition, and qualitatively (ie, present or absent) evaluated using cellulose acetate affinity electrophoresis.

RESULTS

In phenobarbital-treated dogs with high serum total AP activity in the cross-sectional study, the increase was due predominantly to increased activities of the corticosteroid-induced (C-AP) and liver (L-AP) isoenzymes. Prospectively, serum total AP and L-AP activities were significantly higher at 3 weeks, 6 months, and 12 months after the start of phenobarbital treatment compared with pretreatment values. Serum C-AP and bone isoenzyme (B-AP) activities were significantly higher after 6 and 12 months of treatment. B-AP accounted for only a small amount of the total AP activity. No unusual or previously unidentified AP isoenzymes were identified.

CONCLUSIONS

Phenobarbital treatment was associated with increased C-AP and L-AP isoenzyme activities and with a minor increase in B-AP activity. No unique "phenobarbital-induced" isoenzyme was identified. Isoenzyme analysis does not appear to be useful for differentiating between high serum total AP due to phenobarbital therapy and other causes.

The histochemical location of three diagnostic enzymes in the marmoset liver

The histochemical locations of alkaline phosphate (ALP), leucine aminopeptidase (LAP) and gamma glutamyltransferase (GGT) were demonstrated in the liver of the marmoset (Callithrix jacchus). Although all three enzymes were located in cell membranes, the location of LAP was demonstrated by a chromogenic substrate, in the canalicular membrane. GGT was seen in a vascular network, provisionally identified as the peribiliary arterial plexus. Possible diagnostic applications in toxicology are discussed.

Endoprotease in human liver transforming multiple forms of alkaline phosphatase

Two forms of alkaline phosphatase, extracted from human liver and named API1 and API3, are of high molecular mass, but API3 is the larger molecule and is membrane-bound while API1 is smaller and soluble. Enzyme kinetics are identical. It is suggested that API1 is produced from API3 by an endoprotease. We demonstrated the action of an endoprotease in human liver homogenate converting API3 into API1. In the absence of this enzyme no conversion occurred. This enzyme is active at an acidic pH (less than 6.5) in the presence of Ca.. or Mg.. -ions. It is inhibited by traces of EDTA. It is insensitive to diisopropyl fluoro-phosphate, to leupeptin and to reducing or oxidizing chemicals. At alkaline pH (8.6) its activity is rapidly destroyed. The enzyme is stable in acidic buffer. We conclude that API1 is indeed formed from API3 in the living cell by enzymatic conversion.

Clinical use of serum enzymes in liver disease

Some of the many enzymes found in hepatocytes can be measured in the serum and are used as tests of liver function. We now review the current knowledge of their physiology and pathophysiology and outline their clinical usefulness. We divide them into two categories: enzymes that primarily reflect cholestasis, such as the alkaline phosphatase, the 5'-nucleotidase, and the gamma-glutamyl transpeptidase, and those that primarily reflect hepatocellular necrosis, such as the aminotransferases. We also briefly discuss several enzymes of more limited usefulness.

Molecular nature of three liver alkaline phosphatases detected by drug administration in vivo: differences between soluble and membranous enzymes

1. Activities of alkaline phosphatase, liver-membranous, liver-soluble and serum-soluble, were dramatically induced in dogs by treatment with both phenobarbital and brovanexine. The treatment induced a 17-fold increase in membranous, a 155-fold increase in soluble, and a 105-fold increase in serum alkaline phosphatases. 2. There was no difference in the enzymatic behavior of the three forms of alkaline phosphatase, on heat stability, amino acid inhibition and optimum pH. 3. When the three alkaline phosphatases were treated initially with n-butanol, their apparent molecular size was identical. After treatment with phosphatidylinositol-specific phospholipase C, the liver-soluble and serum-soluble alkaline phosphatase were of the same molecular size. Liver-membranous alkaline phosphatase, however, was larger in molecular size than the other two forms, suggesting a difference between soluble and membranous alkaline phosphatase forms. 4. In terms of the sugar moiety of the three alkaline phosphatase forms, the membranous enzyme showed more of the higher affinity fraction and less of the lower affinity fraction of concanavalin A, compared with the soluble enzymes. 5. Consequently, it is possible that the membranous enzyme may be solubilized by an enzyme such as phosphatidylinositol-specific phospholipase C and modify further the sugar moiety of alkaline phosphatase molecules, resulting in serum alkaline phosphatase transfer from the soluble enzyme in liver.

The mechanism of elevated alkaline phosphatase activity after bile duct ligation in the rat

Alkaline phosphatase activity in the liver and intestine increases after bile duct ligation, reportedly by increased enzyme synthesis. To ascertain the mechanism of this increased synthesis in the absence of a cDNA clone encoding the enzyme, we have estimated the concentration of liver and intestinal alkaline phosphatase mRNA by translational analysis. Monospecific antiserum to rat placental alkaline phosphatase was raised. The resulting antiserum precipitated two peptides of 53 and 56 kd after translation of liver poly(A) + RNA. The precipitation of both peptides was blocked by the single 64 kd placental alkaline phosphatase. Processing of the cell-free products by microsomal membranes produced peptides of 62 and 64 kd. Antiserum to rat intestinal alkaline phosphatase also identified two peptides as products of intestinal RNA translation. After bile duct ligation, we confirmed a transient 2-fold increase in alkaline phosphatase activity in the intestine and a more constant 7-fold increase in the liver. However, the alkaline phosphatase mRNA concentration remained unchanged in both organs. We conclude that increased alkaline phosphatase synthesis after bile duct ligation results from an enhanced rate of translation of mRNA.

Rat liver alkaline phosphatases. Evidence hepatocyte and portal triad enzymes differ

Using biochemical and electron microscopic histochemical techniques, we studied membrane-bound alkaline phosphatase activities of rat hepatocytes and portal triads. Activity in portal triads was localized to capillaries surrounding bile ducts (peribiliary plexus) and arterioles. Despite the reputation of alkaline phosphatase as a "biliary enzyme," activity was not observed in bile ducts. Livers were separated into hepatocyte and portal triad fractions with collagenase. Enzyme from hepatocytes migrated faster during electrophoresis and eluted later during anion-exchange chromatography than that from portal triads. Thus, hepatocyte enzyme is more negatively charged (and also possibly smaller) than portal triad enzyme. Twelve hours after bile duct obstruction, new activity appeared on lateral and sinusoidal membranes of hepatocytes; appearance of portal triads did not change with obstruction. Electrophoretic mobilities of the two forms were not altered by obstruction. We conclude that two distinct liver alkaline phosphatases exist, one in hepatocytes, the other in portal triad blood vessels.

Multiple forms of alkaline phosphatases in human liver tissue

Alkaline phosphatases (AP) extracted in the presence of n-butanol from human liver are separated by affinity chromatography on phenylsepharose Cl-4B into two fractions named APII and APIIII. By repeated chromatography, APII was purified to a single enzyme entity with a specific activity of 1,684 kU/g protein. APIIII was purified to a specific activity of 535 kU/g protein. It consisted of only APIIII enzyme activity, but still contained gamma-glutamyltransferase activity. These two forms of AP are different in chromatographic and electrophoretic behaviour, APIIII being a larger molecule than APII. APII and APIIII are very similar in enzyme kinetic behavior, such as substrate activity, thermolability and sensitivity to different inhibitors. It is concluded from these experiments that multiple forms of AP in liver bear identical active centres, the difference is due to a modification of protein residue. It is possible that both are modified forms of one enzyme. Both are different from the AP isoenzyme that appears in serum in cholestatic patients.

Similarities and differences between models of extrahepatic biliary obstruction and complete biliary retention without obstruction in the rat

Biliary obstruction in the rat causes known biochemical and morphological abnormalities. These abnormalities might be caused simply by retention of biliary constituents or might require other factors such as high biliary pressure, interruption of the normal flux of fluid, ions, and other biliary constituents through the hepatocyte and/or changes in intracellular concentrations, binding, and distribution of biliary constituents. We studied a choledochocaval (CC) fistula rat model characterized by complete biliary retention without bile stasis. CC fistula animals were compared with biliary obstructed rats over a 46-hr period. Bile flow and bile acid excretion rate reached high levels by hour 5 and remained high in CC fistula rats. Urinary bile acid excretion, shrinkage of the rapidly exchangeable bile acid pool, and serum bile acid and bilirubin concentrations were equivalent in the two models. Serum cholesterol concentration was higher and serum alkaline phosphatase and 5'-nucleotidase activities lower in obstructed rats. The percentage of beta-muricholate rose and the percentage of cholate decreased in bile acid from obstructed rats. This was associated with increased microsomal 6 beta-hydroxylase activity. Bile acid composition and microsomal 6 beta-hydroxylase activity changed little in CC fistula rats. Morphological examination revealed greater damage in obstructed rat livers. These data indicate that factors other than simple retention of biliary constituents contribute to the abnormalities observed in extrahepatic biliary obstruction.

Biochemistry of alkaline phosphatase isoenzymes

The review discusses the similarities and differences between the common isoenzymatic forms of ALP. Methods for separating, measuring, and purifying the isoenzymes on the basis of these differing properties are described. The evidence is reviewed for the existence of different genes coding for different isoenzymes, and the current state of knowledge is surveyed concerning the location, development, function, and regulation of the isoenzymes. Finally, some unusual forms of ALP which may appear in the circulation are described.

Multiple forms of human intestinal alkaline phosphatase: chemical and enzymatic properties, and circulating clearances of the fast- and slow-moving enzymes

Two forms of alkaline phosphatase (orthophosphoric monoester phosphohydrolase (alkaline optimum, EC 3.1.3.1) have been purified from human small intestine by column chromatography on DEAE-cellulose and tyraminyl derivative affinity gel, and by preparative disc gel electrophoresis. Intestinal phosphatases were electrophoretically separated into two components, fast- and slow-moving enzymes, with apparent molecular weights of 140000 and 168000 and with subunit weights of 68000 and 80000, respectively. Analyses of carbohydrate and amino acid revealed marked differences in the two enzymes. Enzymatic properties and affinities for an anti-blood group antibody were also found to differ. Papain digestion released a hydrophobic small peptide from the slow-moving enzyme and its enzymatic properties resembled those of the fast-moving enzyme. Circulating clearance (T1/2) of the slow- and fast-moving enzymes from adult intestine was found to be 7.5 h and 1.3 h, respectively; that of fetal intestinal enzyme was 2.8 h. Sialidase, sialidase/beta-galactosidase, or sialidase/beta-galactosidase/N-acetyl-beta-glucosaminidase treatment of the fetal enzyme reduced the value to about 40 min. Further, digestion with alpha-fucosidase, alpha-mannosidase or both restored it to nearly the original level. Organ distribution of injected 125I-labelled enzymes indicates that the desialylated hepatic enzyme was selectively distributed in liver, while the degalactosylated intestinal enzyme was incorporated into liver lymph fluid, and small intestine. These results suggest that the pathway of circulating clearance of alkaline phosphatase has several routes.

Alkaline phosphatase isoenzymes in serum of rats during cholestasis

Alkaline phosphatase isoenzymes (API) in serum of rats during cholestasis are investigated. For comparison different membrane systems in liver are damaged. Proliferation of bile canaliculi, sinusoidal area, and endoplasmic reticulum, respectively, is induced by different toxic conditions. It is found that in cholestasis an API5 in serum arises which is not present in serum of normal rats, but can be detected in normal rat liver. Thus, it is not a de novo synthesis of this API. Under the condition connected with a proliferation of bile canaliculi we find this API5 in serum. Under different conditions without proliferation of bile canaliculi we do not find an increase of this API5. We assume, therefore, that API5 in cholestasis is produced by cells of the bile canaliculi rather than by liver parenchymal cells in the sinusoidal area. No difference is found in intra- or extrahepatic cholestasis.

Isolation of a rat liver plasma membrane fraction of probable canalicular origin. Preparative technique, enzymatic profile, composition, and solute transport

A technique currently used for isolation of brush border membranes from renal and intestinal epithelium that involves vigorous tissue homogenization and sedimentation of non-luminal membranes in the presence of Mg2+ has been adapted to rat liver. Liver plasma membranes so prepared consisted almost exclusively of vesicles by electron microscopy, showed some contamination with endoplasmic reticulum and minimal contamination with mitochondria or Golgi by marker enzymes, were highly enriched in alkaline phosphatase, Mg2+-ATPase, and 5'-nucleotidase activity compared with homogenate, and showed little enrichment in (Na+, K+)-ATPase. Comparison of this enzymatic profile with cytochemical studies localizing (Na+, K+)-ATPase and alkaline phosphatase to the sinusoidal/lateral and canalicular membranes, respectively, suggested that these membranes were predominantly of canalicular origin. They had a lower (Na+ + K+)-ATPase specific activity, lower lipid content, and higher cholesterol to phospholipid molar ratio than a conventional plasma membrane preparation believed to be enriched in canaliculi. Moreover, it was possible to measure movement of D-[3H]glucose into an osmotically sensitive space bounded by these membrane vesicles.

Reversal of ethinyl estradiol-induced bile secretory failure with Triton WR-1339

The effects of Triton WR-1339 and phenobarbital on ethinyl estradiol bile secretory failure were examined to determine the mechanism responsible for decreased bile salt excretion. When administered to ethinyl estradiol-treated rats, Triton WR-1339 restored bile salt independent bile flow and maximum taurocholate transport, whereas phenobarbital corrected bile flow only. Ethinyl estradiol decreased the activities of Na(+)-K(+)-ATPase, 5'-nucleotidase, while increasing the activities of Mg(++)-ATPase and alkaline phosphatase. In contrast to these heterogeneous changes in surface membrane enzyme activities, the number and affinity of [(14)C]cholic acid carriers were not altered. When administered in vivo or added directly to surface membrane fractions Triton WR-1339 restored the activities of Na(+)-K(+)-ATPase and Mg(++)-ATPase of rats treated with ethinyl estradiol through a process that did not require protein synthesis (unaffected by cycloheximide). Phenobarbital also restored the activity of Na(+)-K(+)-ATPase to control levels, but, unlike Triton WR-1339 it did not correct the defect responsible for reduced bile salt secretion. Ethinyl estradiol increased the concentration of cholesterol esters in surface membrane fractions. When administered to ethinyl estradiol-treated rats, Triton WR-1339 restored cholesterol ester concentrations to normal, whereas phenobarbital did not. These combined data suggest that decreased or altered bile salt carriers or reduced sodium driving forces resulting from impaired activity of Na(+)-K(+)-ATPase are not responsible for decreased bile salt excretion in ethinyl estradiol-treated rats. It is proposed that the diverse changes in surface membrane function, which are associated with ethinyl estradiol bile secretory failure, may be the result of a generalized alteration in membrane lipid structure.

Studies on alkaline phosphatase isoenzymes in hepatic diseases. Relation to gamma-glutamyltransferase

Isoenzymes of alkaline phosphatase (ALP) and total gamma-glutamyltransferase (gamma GT) have been studied in patients with increased total ALP. Fractionation of alkaline phosphatase yielded clinical information which could not be obtained by determination ALP and gamma GT alone. 1. There was a high degree of correlation between isoALP 1 (biliary band) and total gamma GT. 2. The ALP2 fraction increases after cytolysis in acute and chronic hepatitis. 3. A new ALP4 fraction appears, probably due to fibroblastic activity, in some histological types of cirrhosis.