Naohito Ohmi

Ligandin: An adventure in Liverland

SummaryLigandin is an abundant soluble protein which has at 1/2 of 2–3 days, is induced by many drugs and chemicals, and is stabilized in the absence of thyroid hormone. The protein is strategically concentrated in cells associated with transport and detoxification of many endogenous ligands, such as bilirubin, and exogenous ligands, such as drugs and chemicals. The protein is a dimer in rat liver. Whether the dimer is a primary gene product or at least two genes are involved is not known. The protein has broad, low affinity catalytic activity as a GSH-S-transferase for many ligands having electrophilic groups and hydrophobic domains. It catalyzes formation of GSH conjugates, noncovalently binds some ligands prior to their biotransformation or excretion in bile, and covalently binds other ligands, such as activated carcinogens. Recent studies include the possible role of ligandin in chemical carcinogenesis, diagnosis of inflammatory and neoplastic disease of the liver and kidney, and participation in intracellular transport. Although some of the roles that have been outlined are speculative, any single function is important. The GSH-Stransferases are primitive enzymes and non specific binding proteins but “it is precisely their simplistic design that allows such protean serviceability”.Ligandin illustrates a group of hepatic disposal mechanisms which involve bulk transport of ligands. Although specific uptake and transport mechanisms have been described for several hormones which enter the hepatocyte in small quantities and regulate intermediary metabolism and, possibly, cell maturation, bulk transport of ligands into, through and out of the liver involves mechanisms which accomodate many metabolites, drugs and chemicals of diverse structure. The liver is bathed in sewage which contains what we ingest or are injected with and potentially toxic products of intestinal microorganisms. The chemical formulas of the many substances which are metabolized by the liver provide a horror show of potentially reactive and toxic metabolites, mutagens and carcinogens. Despite this alimentary “Love Canal”, we and our livers do remarkably well. These hepatic disposal mechanisms, as exemplified by ligandin, evolved in ancient times. They are present, albeit sluggishly, in insects and ancient elasmobranchs. Hepatic uptake and removal mechanisms of high capacity, modest affinity and broad substrate range permit us to live in what has probably always been a threatening world.

A Prospective Study of Urinary Ligandin in Patients at Risk of Renal Tubular Injury

The urinary excretion of ligandin, a proximal tubular enzyme and binding protein, was measured by radioimmunoassay in eight normals, six patients receiving radiocontrast media, and six patients in a critical care unit who were considered at high risk for acute renal failure. Ligandinuria was found to occur normally at rates under 5 micrograms/hr. In the patients receiving radiocontrast media, abnormal rates of ligandinuria were found in four patients. In 102 ligandin measurements in the critically ill patients, rates of ligandinuria exceeded normal only once (after contrast media exposure) despite 13 identifiable episodes of potentially nephrotoxic circumstances and two episodes of acute renal failure. Ligandinuria appears more sensitive as a marker for tubular injury from contrast media than from other renal insults.

Hepatocellular Ligandin during N-2-Fluorenylacetamide Carcinogenesis

Ligandin was decreased by 75% as determined immunologically and by glutathione-S-transferase or steroid isomerase activities in rat hepatocellular carcinomas induced by exposure to N-2-fluorenylacetamide. Minor variable differences in ligandin levels were noted between the putative, premalignant nodules induced by this regimen and normal liver.

The effect of limited proteolysis on enzymatic, binding and immunological properties of ligandin

Abstract The peptide mixture obtained from controlled proteolytic digestion of ligandin with proteinase K or subtilisin retained 40% of glutathione-S-transferase and steroid isomerase activities, immunological reactivity and lower affinity bilirubin binding but binding at the primary site was abolished. When these limited proteolytic digests, which had no intact ligandin as determined by SDS gel electrophoresis, were subjected to Sephadex G-75 column chromatography, 40–50% of the peptide fragments were recovered in fractions where intact ligandin eluted. The results suggest that intact ligandin is not required for enzymatic activities, binding of bilirubin at the secondary site, or immunological reactivity; steroid isomerase and glutathione-S-transferase activities are modulated in a parallel manner and may be mediated by the same region of the protein, and primary and secondary binding sites for bilirubin are distinct and independent, despite nicks introduced by proteolysis in ligandins subunits, some of the fragments remain associated under non-denaturing conditions and the susceptibility of the two subunits to the proteases is different.