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T Litman

Publications and source records attributed to T Litman.

28 records · Page 2Linked to original sources
PubMed1997

ATPase activity of P-glycoprotein related to emergence of drug resistance in Ehrlich ascites tumor cell lines.

We have characterized the ATPase activity of a sensitive and five progressively daunorubicin resistant Ehrlich ascites tumor cell lines passaged in mice. For the nine different modulators of drug resistance that we have studied, the ATPase activity first rose with the modulator concentration and then declined. We analyzed the ATPase activity profiles in terms of an activation constant and an inhibition constant for each of the nine drugs and six cell lines. In this series of cell lines, the drug-stimulatable ATPase activity was directly proportional to the amount of P-glycoprotein. Pumping of daunorubicin was also correlated with the amount of P-glycoprotein, except that, for a highly passaged line more daunorubicin was pumped than could be accounted for by the content of P-glycoprotein. Between the 12th and the 36th passage an additional source of resistance emerged, which was not correlated with P-glycoprotein. Pumping of daunorubicin was negatively correlated with the cell volume for the different lines.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Structure-activity relationships of P-glycoprotein interacting drugs: kinetic characterization of their effects on ATPase activity.

We have determined the kinetic parameters for stimulation and inhibition by 34 drugs of the P-glycoprotein ATPase in membranes derived from CR1R12 Chinese hamster ovary cells. The drugs chosen were sets of calmodulin antagonists, steroids, hydrophobic cations, hydrophobic peptides, chemotherapeutic substrates of P-glycoprotein, and some other drugs with lower affinity for P-glycoprotein. We studied how these kinetic parameters correlated with the partition coefficient and the Van der Waals surface area of the drugs. The maximum velocity of ATPase stimulation decreased with surface area and showed a suggestion of a maximum with increasing partition coefficient. The affinity of these drugs for P-glycoprotein showed no significant correlation with partition coefficient but was highly correlated with the surface area suggesting that binding between modulators and P-glycoprotein takes place across a wide interaction surface on the protein.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Competitive, non-competitive and cooperative interactions between substrates of P-glycoprotein as measured by its ATPase activity.

We have studied the interaction between verapamil and other modulators of the P-glycoprotein ATPase from membranes of CR1R12 Chinese hamster ovary cells. Four major categories of interaction were identified. (i) Non-competitive inhibition of verapamil's stimulation of enzyme activity was found with vanadate. (ii) Competitive inhibition of the ATPase was found for the pair verapamil and cyclosporin A. (iii) Allosteric inhibition with an increase in the Hill number for verapamil was found in the cases of daunorubicin, epirubicin, gramicidin S and D, vinblastine, amiodarone, and colchicine. (iv) Cooperative stimulation of verapamil-induced ATPase activity was found with progesterone, diltiazem, amitriptyline, and propranolol. At high levels, progesterone and verapamil mutually enhanced each other's inhibitory action on the ATPase. Our data show that the substrate binding behavior of P-glycoprotein is complex with more than one binding site being present. This information could form the basis for the development of improved modulators of P-glycoprotein.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Water transport by the Na+/glucose cotransporter under isotonic conditions.

Solute cotransport in the Na+/glucose cotransporter is directly coupled to significant water fluxes. The water fluxes are energized by the downhill fluxes of the other substrates by a mechanism within the protein itself. In the present paper we investigate the Na+/glucose cotransporter expressed in Xenopus oocytes. We present a method which allows short-term exposures to sugar under voltage clamp conditions. We demonstrate that water is cotransported with the solutes despite no osmotic differences between the external and intracellular solutions. There is a fixed ratio of 195:1 between the number of water molecules and the number of Na+ ions transported, equivalent to 390 water molecules per glucose molecule. Unstirred layer effects are ruled out on the basis of experiments on native oocytes incubated with the ionophores gramicidin D or nystatin.

Animals↗
PubMed1997

Drug accumulation in the presence of the multidrug resistance pump: dissociation between verapamil accumulation and the action of P-glycoprotein.

We studied the interaction between the multidrug transporter, P-glycoprotein, and two compounds that interact with it: vinblastine, a classical substrate of the pump, and verapamil, a classical reverser. Steady-state levels of accumulation of these two drugs were determined in a multidrug resistant P388 leukemia cell line, P388/ADR. The time course of accumulation of these drugs, and the effect of energy starvation and the presence of chloroquine on the level of their steady-state accumulation were quite disparate. Vinblastine inhibited the accumulation of verapamil whereas it enhanced the accumulation of daunomycin, another classic substrate of P-glycoprotein. Verapamil did not compete with the intracellular binding sites of vinblastine. In all these aspects, vinblastine behaved as a typical substrate of P-glycoprotein but verapamil did not. Our data suggest that verapamil is a reverser of P-glycoprotein but that its intracellular accumulation is not affected by this membrane-bound transporter.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1977

Ski injuries.

A group of 147 injuried skiers seen in the emergency room of a midwestern hospital was studied. There were almost twice as many males as females, and lower-extremity injuries outnumbered upper-extremity injuries two to one. Leg injuries tended to occur in beginners whose bindings failed to release. Upper-extremity injuries were prevalent in better skiers whose bindings released properly. Frequent binding checks only slightly improved the likelihood of proper binding function.

Adolescent↗
PubMedDate not supplied

Cellular and biophysical evidence for interactions between adenosine triphosphate and P-glycoprotein substrates: functional implications for adenosine triphosphate/drug cotransport in P-glycoprotein overexpressing tumor cells and in P-glycoprotein low-level expressing erythrocytes.

P-glycoprotein is involved with the removal of drugs, most of them cations, from the plasma membrane and cytoplasm. Pgp is also associated with movement of ATP, an anion, from the cytoplasm to the extracellular space. The central question of this study is whether drug and ATP transport associated with the expression of Pgp are in any way coupled. We have measured the stoichiometry of transport coupling between drug and ATP release. The drug and ATP transport that is inhibitable by the sulfonylurea compound, glyburide (P. E. Golstein, A. Boom, J. van Geffel, P. Jacobs, B. Masereel, and R. Beauwens, Pfluger's Arch. 437, 652, 1999), permits determination of the transport coupling ratio, which is close to 1:1. In view of this result, we asked whether ATP interacts directly with Pgp substrates. We show by measuring the movement of Pgp substrates in electric fields that ATP and drug movement are coupled. The results are compatible with the view that substrates for Pgp efflux are driven by the movement of ATP through electrostatic interaction and effective ATP-drug complex formation with net anionic character. This mechanism not only pertains to drug efflux from tumor cells overexpressing Pgp, but also provides a framework for understanding the role of erythrocytes in drug resistance. The erythrocyte consists of a membrane surrounding a millimolar pool of ATP. Mammalian RBCs have no nucleus or DNA drug/toxin targets. From the perspective of drug/ATP complex formation, the RBC serves as an important electrochemical sink for toxins. The presence in the erythrocyte membrane of approximately 100 Pgp copies per RBC provides a mechanism for eventual toxin clearance. The RBC transport of toxins permits their removal from sensitive structures and ultimate clearance from the organism via the liver and/or kidneys.

ATP Binding Cassette Transporter, Subfamily B, Mem↗