Identification of the platelet glycoprotein IIb/IIIa complex as a target antigen in primary biliary cirrhosis-associated autoimmune thrombocytopenia. Evidence that platelet-reactive autoantibodies can also bind to the mitochondrial antigen M2

A 67-year-old woman with a 4-year history of primary biliary cirrhosis (PBC) unexpectedly developed autoimmune thrombocytopenia. The platelet-bound IgG antibody was eluted from the patient's platelets to determine the platelet target antigen. The autoantibodies were found to precipitate the platelet glycoprotein complex IIb/IIIa of autologous and allogeneic platelets. A further precipitate of 70 kDa was detectable under reducing conditions. In addition, platelet-reactive antibodies bound to the 70 kDa mitochondrial antigen M2. No cross-absorption studies were performed to confirm that a single antibody reacted with both antigens. Computer analysis of published peptide sequences of the mitochondrial protein and the platelet GPIIb/IIIa complex showed partial amino acid sequence homology suggesting the possibility of a common antibody binding site. These findings suggest a relationship between the development of autoimmune thrombocytopenia in PBC and the underlying liver disease.

Determination of DR beta 1 alleles of DR4/Dw subtypes by oligonucleotide probing

The extensive Dw polymorphism of HLA-D has, to date, been defined using cumbersome and lengthy techniques involving T-cell proliferative reactions. These techniques have not necessarily provided information about the genotype at any single locus of HLA-D because the products of more than a single locus of HLA-D (e.g., DR beta 1 and DR beta 2 of the DR2 haplotypes and in some cases DQ) can contribute to T-cell proliferation and thus to assignment of Dw phenotype. We have recently reported that DR beta 1 alleles relating to the Dw polymorphism can vary by only a single, or a very few, nucleotide differences; the products of this locus are recognized in allo- and restricted responses. Given the importance of defining this polymorphism, we have used specific oligonucleotides as hybridization probes to differentiate between these very closely related alleles. This approach provides the basis for a rapid method for HLA-D typing which, as additional sequence information becomes available, will likely be generally applicable. In addition, oligonucleotide probing allows definition of a gene transcript from an individual locus of HLA-D rather than the phenotype results provided by Dw typing.

Hydrodynamic properties of the chromaffin granule hydrogen ion pumping adenosinetriphosphatase

We have determined the hydrodynamic properties of detergent-solubilized ATPase, which is coupled to H+ pumping in bovine adrenal chromaffin granules, by sedimentation equilibrium centrifugation and gel permeation chromatography. The protein solubilized with detergent containing phosphatidylserine sediments as a particle of 264,000 daltons and partial specific volume 0.829 cm3/g. Assuming a protein v of 0.73 and using the v measured for detergent and lipid mixed micelles of 0.93 cm3/g, we calculated that the protein component has a mass of 134,000 daltons and that the equivalent of approximately 1.5 micelles of detergent are bound per particle. The particle exhibits a Stokes radius of 43 A, which, together with the calculated particle volume, indicates an axial ratio close to 1. We conclude that the ATPase is an intrinsic membrane protein with a structure very different from that of mitochondrial F1F0 ATPase.

Characterization of native and reconstituted hydrogen ion pumping adenosinetriphosphatase of chromaffin granules

The ATP-dependent H+ pump from adrenal chromaffin granules is, like the platelet-dense granule H+ pump, essentially insensitive to the mitochondrial ATPase inhibitors sodium azide, efrapeptin, and oligomycin and also insensitive to vanadate and ouabain, agents that inhibit the Na+,K+-ATPase. The chromaffin granule H+ pump is, however, sensitive to low concentrations of NEM (N-ethylmaleimide) and Nbd-Cl (7-chloro-4-nitro-2,1,3-benzoxadiazole). These transport ATPases may thus belong to a new class of ATP-dependent ion pumps distinct from F1F0-and phosphoenzyme-type ATPases. Comparisons of ATP hydrolysis with ATP-dependent serotonin transport suggest that approximately 80% of the ATPase activity in purified chromaffin granule membranes is coupled to H+ pumping. Most of the remaining ATPase activity is due to contaminating mitochondrial ATPase and Na+,K+-ATPase. When extracted with cholate and octyl glucoside, the H+ pump is solubilized in a monodisperse form that retains NEM-sensitive ATPase activity. When reconstituted into proteoliposomes with crude brain phospholipid, the extracted enzyme recovers ATP-dependent H+ pumping, which shows the same inhibitor sensitivity and nucleotide dependence as the native pump. These data demonstrate that the predominant ATP hydrolase of chromaffin granule membrane is also responsible for ATP-driven amine transport and granule acidification in both native and reconstituted membranes.

The hydrogen ion-pumping adenosine triphosphatase of platelet dense granule membrane. Differences from F1F0- and phosphoenzyme-type ATPases

Using a coupled transport assay which detects only those ATPase molecules functionally inserted into the platelet dense granule membrane, we have characterized the inhibitor sensitivity, substrate specificity, and divalent cation requirements of the granule H+ pump. Under identical assay conditions, the granule ATPase was insensitive to concentrations of NaN3, oligomycin, and efrapeptin which almost completely inhibit ATP hydrolysis by mitochondrial membranes. The granule ATPase was inhibited by dicyclohexylcarbodiimide but only at concentrations much higher than those needed to maximally inhibit mitochondrial ATPase. Vanadate (VO3-) ion and ouabain also failed to inhibit granule ATPase activity at concentrations which maximally inhibited purified Na+,K+-ATPase. Two alkylating agents, 7-chloro-4-nitrobenz-2-oxa-1,3-diazole and N-ethylmaleimide both completely inhibited H+ pumping by the granule ATPase under conditions where ATP hydrolysis by mitochondrial membranes or Na+,K+-ATPase was hardly affected. These results suggest that the H+-pumping ATPase of platelet granule membrane may belong to a class of ion-translocating ATPases distinct from both the phosphoenzyme-type ATPases present in plasma membrane and the F1F0-ATPases of energy-transducing membranes.

Hydrogen ion cotransport by the renal brush border glutamate transporter

Sodium ion dependent glutamate transport into rabbit renal brush border membrane vesicles is stimulated by low external pH and inhibited by low internal pH. Imposition of a pH difference (delta pH) (interior alkaline) across the vesicle membrane drives glutamate accumulation in the absence of other driving forces. This process requires Na+ but is not due to generation of an Na+ gradient in response to delta pH. Internal K+ stimulates both the rate and extent of glutamate accumulation, although K+ is not absolutely required for transport. Internal H+ inhibits the rate of glutamate accumulation by decreasing this K+ stimulation. Conversely, external K+ inhibits glutamate influx, and this inhibition can be overcome by lowering the external pH. These results indicate that H+ is cotransported with glutamate and suggest the possibility that, in the absence of internal K+, H+ can also fulfill the requirement for a countertransported ion.

The serotonin transporter-imipramine "receptor"

The platelet plasma membrane serotonin transporter requires Na+ for two reactions, serotonin transport and imipramine binding. Although imipramine binding has been thought to reflect the same process required for serotonin binding prior to transport (Talvenheimo, J., Nelson, P.J., and Rudnick, G. (1979) J. Biol. Chem. 254, 4631-4635), binding and transport display markedly different responses to Na+. Imipramine binding (and competitive inhibition of transport) apparently requires two sodium ions which bind with a KD of 300 +/- 70 meq/liter. The total number of sites (Bmax) is the same at all Na+ concentrations, but the affinity for imipramine increases from 7.3 x 10(6) M-1 at 20 meq/liter to 110 x 10(6) M-1 at 200 meq/liter. Na+ acts, at least in part, by decreasing the rate of imipramine dissociation from its binding site. Serotonin binding displaces imipramine from its site on the membrane. In contrast to imipramine binding, this displacement is a simple, hyperbolic function of Na+ concentration with a KD for Na+ of 400 +/- 100 meq/liter, which suggests that only one Na+ is required. Serotonin transport is also much less responsive to Na+ concentration. Over the same concentration range in which the affinity for imipramine increases 15-fold, the affinity for serotonin increases only 2-fold. Despite the lack of Na+ effect on the Bmax for imipramine binding, the Vmax for serotonin transport increases as a simple saturable function of Na+ with a KM (Na+) of 52 meq/liter. Thus, substrate translocation as well as binding requires Na+. Since serotonin is cotransported with Na+, the serotonin gradient accumulated depends on the coupling stoichiometry and the magnitude of the Na+ gradient imposed. From the response of the serotonin gradient to imposed Na+ gradients, we calculated a serotonin:Na+ cotransport stoichiometry of 0.9. Taken together, the results suggest that serotonin and imipramine bind either to the same site or to mutually exclusive sites, but maximal imipramine binding requires two sodium ions, while maximal serotonin binding and translocation requires only one.

Mammalian enzymes of trimethyllysine conversion to trimethylaminobutyrate

The biosynthesis of carnitine proceeds from trimethyllysine (TML) by beta-hydroxylation by a liver or kidney mitochondrial enzyme, which requires oxygen, alpha-ketoglutarate, ferrous iron, and ascorbate. This dioxygenase is rapidly inactivated by preincubation with Fe2+, but not Fe3+. The evidence suggests that superoxide anion is involved in the hydroxylation. beta-Hydroxytrimethyllysine undergoes aldol cleavage to glycine and trimethylaminobutyraldehyde under the influence of serine hydroxymethyltransferase and possibly a specific aldolase. The next step, the aldehyde oxidation, is catalyzed by a specific NAD-dependent aldehyde dehydrogenase from liver cytosol. The product, trimethylaminobutyrate, is then hydroxylated by a cytosolic dioxygenase to carnitine. This enzyme, which has the same cofactor requirements as TML hydroxylase, is found in the liver of all species examined, but is absent from the kidney of some species.

The role of chloride ion in platelet serotonin transport

Chloride ion is required for platelet serotonin transport. This anion requirement is relatively specific for Cl-. Only Br- fully substitutes, while SCN-, NO2-, and NO3- are much less effective. Serotonin influx requires external Cl-, and efflux requires internal Cl-. Although internal Cl- is not required for transport and has little effect on the initial rate of influx, it decreases the steady state level of serotonin accumulation. Chloride ion does not merely fulfill a need for a permanent ion since it is required for exchange of internal with external serotonin. Moreover, serotonin accumulation still requires external Cl- when a K+ diffusion potential (interior negative) is imposed across the vesicle membrane with valinomycin. Furthermore, Cl- lowers the Km and raises the Vmax for transport, suggesting direct effects on the transporter. These results strongly suggest serotonin-Cl- co-transport across the platelet plasma membrane.

Anion-dependent sodium ion conductance of platelet plasma membranes

External stimulate Na+ efflux from platelet plasma membrane vesicles. Efflux is apparently electrogenic since K+ diffusion potentials induced with valinomycin (interior positive) accelerate and potentials of the opposite polarity (interior negative) inhibit. In the presence of stimulatory anions, voltage-dependent Na+ efflux is much faster than Na+-Na+ exchange in the absence of an induced membrane potential. Anions stimulate voltage-dependent efflux in the following order: SCN- greater than I- greater than NO3- greater than Br- approximately acetate approximately Cl- greater than F- approximately SO42- greater than HPO42-, gluconate, and isethionate. Thiocyanate, the most stimulatory anion, increases Na+ efflux 20-fold in the presence of a membrane potential (interior positive). Stimulation of efflux by Cl- is a saturable phenomenon with a K0.5 of 41 mM and a maximal 2-3-fold stimulation over the basal level of efflux. Neither basal nor valinomycin-stimulated efflux was influenced by the presence of the platelet-aggregating agents thrombin, epinephrine, or ADP in the presence of fibrinogen.

Effect of ascorbic acid deficiency on the in vivo synthesis of carnitine

The effects of ascorbate deficiency on carnitine biosynthesis was investigated in young male guinea pigs. Liver and kidney carnitine levels were not affected by the deficiency, but scorbutic animals had 50% less carnitine in heart and skeletal muscle than control animals. Labeled carnitine precursors, 6-N-tri-methyl-L-lysine and 4-N-trimethylaminobutyrate, both of which require ascorbate for their enzymatic hydroxylation, were injected into the vena cava of control, pair-fed and scorbutic animals. The distribution of isotope in compounds present in the liver and kidney after 1 h was determined. The uptake of trimethyllysine by the liver was less than 2% in 1 h, while the kidney took up approx. 20% of the 14C. Control and pair-fed animals converted trimethyllysine to kidney trimethylaminobutyrate 8--10 times as well as did scorbutic animals. Trimethylaminobutyrate hydroxylase, present in the liver but almost absent from the kidney, converted nearly all of substrate taken up by the liver to carnitine in both the scorbutic and control animals.

Evidence for two distinct serotonin transport systems in platelets

Two transport systems for serotonin were measured independently in osmotic lysates of porcine platelets. After pre-equilibration with potassium phosphate and dilution into NaCl medium, only the plasma membrane transport system was observed. Under these conditions, serotonin accumulation is inhibited by imipramine, but not by reserpine or norepinephrine, and requires high (100 meq/liter) concentrations of Na+. In the presence of ATP at low (10 meq/liter) concentrations of Na+, only the transport system of the intracellular serotonin storage organelles was observed. Under these conditions, transport is inhibited by reserpine, norepinephrine, and epinephrine, but not by imipramine, and requires either ATP or an artificially imposed pH gradient (interior acidic) across the organelle membrane. These and other results suggest that accumulation of serotonin in platelet storage organelles is coupled predominantly to the pH gradient across the membrane.

Mechanism of imipramine inhibition of platelet 5-hydroxytryptamine transport

Plasma membrane vesicles isolated from porcine blood platelets take up approximately 8 to 15 pmol of [3H]imipramine per mg of membrane protein. This apparent binding requires Na+ in the external medium and is reversed by 5-hydroxytryptamine and fluoxetine. The apparent KD for imipramine uptake is 23 nM, which agrees well with the KI for competitive inhibition of 5-hydroxytryptamine transport by imipramine. In contrast to 5-hydroxytryptamine transport, imipramine uptake is not dependent on transmembrane Na+ and K+ gradients and is insensitive to ionophores such as nigericin and gramicidin which dissipate these gradients. Although 5-hydroxytryptamine rapidly and competitively displaces imipramine from membrane vesicles, imipramine does not cause 5-hydroxytryptamine efflux and inhibits 5-hydroxytryptamine exchange. These results are consistent with the proposal that imipramine binds to the substrate site of the 5-hydroxytryptamine transporter but cannot be transported.

Reconstitution of 5-hydroxytryptamine transport from cholate-disrupted platelet plasma membrane vesicles

The bile acid cholate disrupts platelet plasma membrane vesicles and inactivates 5-hydroxytryptamine transport activity. The transporter appears not to be solubilized by this procedure, but remains associated with a high molecular weight aggregate. Merely removing cholate does not restore the vesicular structure or transport activity, but in the presence of added soybean phospholipids, cholate removal generates proteoliposomes which accumulate 5-hydroxytryptamine. This reconstituted transport activity is similar in its ionic requirements and inhibitor sensitivity to transport in native membrane vesicles and intact platelets.

Platelet 5-hydroxytryptamine transport, an electroneutral mechanism coupled to potassium

Transport of 5-hydroxytryptamine into plasma membrane vesicles isolated from porcine blood platelets is stimulated when a potassium gradient (in greater than out) is imposed across the vesicle membrane. This stimulation occurs in the absence of measurable electrical potential across the membrane. Addition of valinomycin induces a membrane potential of approximately 50 mV (interior negative) as estimated by uptake of the lipophilic cation triphenylmethylphosphonium, but has surprisingly little effect on 5-hydroxytryptamine transport. Addition of 2,4-dinitrophenol dissipates the valinomycin-induced membrane potential. In the absence of valinomycin, 2,4-dinitrophenol has no effect on 5-hydroxytryptamine transport but valinomycin and 2,4-dinitrophenol together inhibit transport, probably by dissipation of the K+ gradient. These results are consistent with an electroneutral mechanism in which 5-hydroxytryptamine influx is directly coupled to potassium ion efflux and argue against an electrogenic mechanism in which there is a net influx of positive charge with 5-hydroxytryptamine.