Gerhart Kurz

Carrier-mediated transport in the hepatic distribution and elimination of drugs, with special reference to the category of organic cations.

Carrier-mediated transport of drugs occurs in various tissues in the body and may largely affect the rate of distribution and elimination. Saturable translocation mechanisms allowing competitive interactions have been identified in the kidneys (tubular secretion), mucosal cells in the gut (intestinal absorption and secretion), choroid plexus (removal of drug from the cerebrospinal fluid), and liver (hepatobiliary excretion). Drugs with quaternary and tertiary amine groups represent the large category of organic cations that can be transported via such mechanisms. The hepatic and to a lesser extent the intestinal cation carrier systems preferentially recognize relatively large molecular weight amphipathic compounds. In the case of multivalent cationic drugs, efficient transport only occurs if large hydrophobic ring structures provide a sufficient lipophilicity-hydrophilicity balance within the drug molecule. At least two separate carrier systems for hepatic uptake of organic cations have been identified through kinetic and photoaffinity labeling studies. In addition absorptive endocytosis may play a role that along with proton-antiport systems and membrane potential driven transport may lead to intracellular sequestration in lysosomes and mitochondria. Concentration gradients of inorganic ions may represent the driving forces for hepatic uptake and biliary excretion of drugs. Recent studies that aim to the identification of potential membrane carrier proteins indicate multiple carriers for organic anions, cations, and uncharged compounds with molecular weights around 50,000 Da. They may represent a family of closely related proteins exhibiting overlapping substrate specificity or, alternatively, an aspecific transport system that mediates translocation of various forms of drugs coupled with inorganic ions. Consequently, extensive pharmacokinetic interactions can be anticipated at the level of uptake and secretion of drugs regardless of their charge.

Transport systems for amphipathic compounds in normal and neoplastic hepatocytes

Photoaffinity labeling of plasma membrane subfractions from liver and of intact liver tissue with a photolabile bile salt derivative, the sodium salt of (7,7-azo-3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-aminoethanesulfonic acid, revealed that the hepatobiliary transport of bile salts is accomplished by transport systems different for sinusoidal uptake and canalicular secretion. Polypeptides with apparent Mr values 54,000 and 48,000 interact with bile salts at sinusoidal membrane, whereas a polypeptide with an apparent Mr of 100,000 is involved in bile salt secretion through the canalicular membrane. Photoaffinity labeling with photolabile derivatives of uncharged and cationic compounds provided evidence that the sinusoidal membrane polypeptides exhibit a broad binding specificity. Photoaffinity labeling studies and kinetic studies suggest that hepatic uptake of different amphipathic anions, uncharged compounds and even of cations is mediated by the sinusoidal transport systems which are involved in the uptake of bile salts. Relatively little is known about the specificity of the canalicular bile salt transport system. The fluorescent bile salt derivative, the sodium salt of (N-[7-(4-nitrobenzo-2-oxa-1,3-diazol)]-3 beta-amino-7 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-aminoethanesulfonic acid, is readily taken up into the hepatocytes of all acinar zones and may be used for the evaluation of the functional state of bile salt transport by fluorescence microscopy. Fluorescent microscopic studies with the fluorescent bile salt derivative showed that ascites hepatoma AS 30D cells do not have the ability to take up bile salts and demonstrated the absence of hepatobiliary bile salt transport in the solid Morris hepatoma 7777. Photoaffinity labeling studies revealed that in both tumor cell models, in hepatoma AS 30D and in Morris hepatoma 7777, the plasma membranes were devoid of the polypeptides having affinities to bile salts and amphipathic cations. A slight labeling of bile salt binding membrane polypeptides in plasma membranes from Morris hepatomas 9618A and TC 5123 opens the possibility to study transport in neoplastic hepatocytes.

The ineffectiveness of analogs of d-galactal as competitive inhibitors of, and substrates for, β-d-galactosidase from Escherichia coli

2,6-Anhydro-3-deoxy-aldehydo-D-lyxo-hept-2-enose (7) and 2,6-anhydro-3-deoxy-D-lyxo-hept-2-enitol (8) were synthesized as half-chair analogs of D-galactal (1). As 1 is a strong inhibitor of, as well as a substrate for, beta-D-galactosidase from Escherichia coli, the same properties were expected for 7 and 8; however, both were ineffective. This result, together with those of other authors, allows speculative conclusions on the tight binding of 1 to the enzyme only, when water or an alcohol is bound as a co-substrate.

Preparation and properties of some photolabile sugar derivatives for affinity labelling

Abstract Sugar derivatives carrying various photolabile groups at various positions have been synthesised, namely, 4-azido-4-deoxy- d -galactose, 4-azi-4-deoxy- d -xylo-hexopyranose (4), 3,7-anhydro-2-azi-1,2-dideoxy- d -glycero- l -manno-octitol (6), and 3-azi-1-methoxybutyl β- d -glucopyranoside (8), and 4-(2-diazo-3,3,3-trifluoropropionyl)- d -glucose. They were tested for their applicability in the photoaffinity labelling of sugar-binding proteins, and the best results with regard to wavelength of irradiation and rate of photolytic decay were obtained with the diazirino derivatives 4, 6, and 8.

Photoaffinity labeling studies of the rat renal sodium/bile salt cotransport system

The uptake of a photolabile taurocholate derivative, (7,7-azo-3 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-aminoethanesulfonate, 7,7-azo-TC, into rat renal brush-border membrane vesicles was stimulated by Na+ and inhibited by taurocholate indicating an interaction with the Na+/bile salt cotransport system. Irradiation of membrane vesicles in the presence of 7,7-azo-TC inhibited Na+-dependent taurocholate uptake irreversibly. Photoaffinity labeling with [3H]7,7-azo-TC resulted in a predominant incorporation of radioactivity into a polypeptide with apparent molecular weight of 99,000. These results suggest that the proteins involved in Na+/bile salt cotransport are similar in renal and ileal brush-border membranes, but differ from those in hepatocytes.

Investigations on the hepatic uptake systems for organic cations with a photoaffinity probe of procainamide ethobromide

Azido procainamide methoiodide (APM), a photolabile derivative of the transport model compound procainamide ethobromide (PAEB), shows a close resemblance to PAEB from a physicochemical point of view. Like PAEB it is effectively taken up by the liver and excreted into bile. Kinetics of the uptake of APM in isolated hepatocytes revealed that in addition to a non-saturable process, two saturable uptake systems are involved (Km1 = 3 microM, Vmax1) = 80 pmol/min/10(6) cells, Km2 = 100 microM, Vmax2 = 130 pmol/min x 10(6) cells). The uptake rate of APM was inhibited markedly in the presence of other organic cations. Organic anions and uncharged compounds generally had no inhibitory effect on the APM uptake. These results support the theory that there is a separate hepatic uptake system for organic cations like APM. Photoaffinity labeling of intact hepatocytes as well as plasma membrane sub-fractions enriched with sinusoidal domains disclosed two major binding polypeptides with apparent M(r) of 48,000 and 72,000. Such labeling patterns were not observed in membranes from hepatoma cells that are deficient in organic solute uptake. Differential photoaffinity labeling with other cationic compounds such as tributylmethyl ammonium and d-tubocurarine reduced the incorporation of APM in these polypeptides. The 48- and 72-kDa proteins might be involved in carrier-mediated transport of type I organic cations at the hepatic uptake level.

The acinar location of the sodium-independent and the sodium-dependent component of taurocholate uptake: A histoautoradiographic study of rat liver

The acinar location of tritium-labelled taurocholate taken up from sodium-containing and sodium-free perfusion media in isolated perfused rat liver was made visible by means of histoautoradiography on cryoslices at low, medium and high bile salt concentrations. In antegrade perfusion studies, in the presence of sodium, with taurocholate concentrations of 1 and 20 microM, respectively, the silver grain label was mainly restricted to acinar zones 1. At an 80 microM concentration, zones 2 and 3 were also labelled but with decreasing intensities towards the terminal hepatic venules. At 120 microM taurocholate, all acinar zones were nearly equally labelled. In the absence of sodium, antegrade liver perfusions with 1, 20 and 120 microM taurocholate resulted in an almost homogenous labelling of all acinar zones and retrograde perfusions in a silver grain density slightly decreasing towards the terminal portal venules. The results indicate that hepatocytes of all acinar zones are capable of taking up taurocholate by both a sodium-dependent and a sodium-independent pathway. The contribution of the sodium-independent uptake to the overall uptake of taurocholate increases with increasing zonal recruitment at higher concentrations. The sodium-dependent uptake, however, is always dominant.

Identification and Function of Bile Salt Binding Polypeptides of Hepatocyte Membrane

In the course of their enterohepatic circulation bile salts are taken up through the sinusoidal and secreted through the canalicular membrane of the hepatocyte. In these different membrane domains different bile-salt- binding polypeptides are present, as demonstrated unequivocally by photo-affinity-labelling studies (Kramer et al., 1982; Buscher et al., 1987; Fricker et al., 1987). All these binding polypeptides could somehow be involved in hepatobiliary bile salt transport.

Mechanisms for Hepato-Biliary Transport of Cationic Drugs Studied with the Intact Organ and on the Membrane Level

About seventy percent of the commonly used drugs belong to the category of (potential) organic cations mainly being quaternary, tertiary, or secondary amines. Quaternary ammonium groups with the nitrogen center linked to four carbon atoms are permanently charged at physiological pH, whereas the tertiary amines become positively charged by acceptance of a proton depending on pH and pK value of the basic group. Cationic drugs can pass membranes by carrier-mediated transport and such processes have been characterized in the kidney tubular cells (Schanker, 1972; Somogyi, 1987; Weiner, 1985) explaining secretion of basic drugs from blood into the urine, in intestine as related to secretion of organic cations from blood into the intestinal lumen (Lauterbach, 1984), in the choroid plexus as a mechanism to remove cationic compounds from the liquor to blood (Schanker, 1972) as well as in the liver mediating hepato-biliary excretion of cationic drugs (Klaassen and Watkins, 1984; Meijer, 1977; 1987; Schanker, 1972).

Stereospecificity of hydrogen transfer catalyzed by d-galactose dehydrogenase from Pseudomonas saccharophila and Pseudomonas fluorescens

Abstract d -Galactose dehydrogenase ( d -galactose: NAD + 1-oxidoreductase EC 1.1.1.48) from Pseudomonas saccharophila and from Pseudomonas fluorescens is shown to be B-side stereo-specific, using d -[1- 3 H]galactose as well as [4- 3 H]NAD + as substrates.