Effect of serum, alpha-1 acid glycoprotein, lipoproteins and albumin on human mononuclear leucocyte beta-adrenoceptors

Human alpha-1-glycoprotein and its interactions with drugs

For about half a century, the binding of drugs to plasma albumin, the "silent receptor," has been recognized as one of the major determinants of drug action, distribution, and disposition. In the last decade, the binding of drugs, especially but not exclusively basic entities, to another plasma protein, alpha 1-acid glycoprotein (AAG), has increasingly become important in this regard. The present review points out that hundreds of drugs with diverse structures bind to this glycoprotein. Although plasma concentration of AAG is much lower than that of albumin, AAG can become the major drug binding macromolecule in plasma with significant clinical implications. Also, briefly reviewed are the physiological, pathological, and genetic factors that influence binding, the role of AAG in drug-drug interactions, especially the displacement of drugs and endogenous substances from AAG binding sites, and pharmacokinetic and clinical consequences of such interactions. It can be predicted that in the future, rapid automatic methods to measure binding to albumin and/or AAG will routinely be used in drug development and in clinical practice to predict and/or guide therapy.

Evidence for inverse regulation of high and low affinity binding sites for (-)125iodocyanopindolol in human mononuclear leucocytes during epinephrine infusion

As we have shown earlier (-)125Iodocyanopindolol (125ICYP) binding to beta-adrenoceptors (beta-AR) in human mononuclear leucocytes (MNL) yields evidence for the existence of high affinity (Bhiaff) and low affinity (Bloaff) binding sites. We studied the regulation of these 2 classes of binding sites during 240 min of (-)-epinephrine (EPI) infusion (0.1 microgram/kg/min) (n = 8) in male healthy volunteers. Saturation experiments were performed on MNL membranes with 125ICYP over a large concentration range (1-550 pmol/l). Binding parameters were calculated by computer analysis assuming 2 classes of binding sites. We found a preinfusion value of 830 +/- 50 [sites/cell] (KD = 1.5 +/- 0.2 pmol/l) of Bhiaff binding sites and 5210 +/- 510 [sites/cell] (KD = 420 +/- 80 pmol/l) of Bloaff. During EPI infusion we observed biphasic modulation of the Bhiaff and an inverse modulation of the Bloaff. After 40 min of EPI Bhiaff increased to 1970 +/- 280 [sites/cell] (KD = 4.2 +/- 0.8 pmol/l), whereas Bloaff decreased to 2720 +/- 280 [sites/cell] (KD = 140 +/- 70 pmol/l); despite constant plasma epinephrine concentration (PEC) after 240 min of EPI Bhiaff changed to 1310 +/- 240 [sites/cell] (KD = 2.8 +/- 1.0 pmol/l) vs. 4370 +/- 760 [sites/cell] (KD = 190 +/- 100 pmol/l) Bloaff. These results suggest an interdependent inverse modulation of the 2 classes of binding sites for 125ICYP on MNL during EPI infusion.

Growth, differentiation and the beta-adrenergic signal system of HL-60 cells. Characterization in a medium with insulin as the only added protein

The purpose of the present investigation was to define experimental conditions for studies on growth, differentiation and the beta-adrenergic signal system of HL-60 cells. The cell medium was completely devoid of added proteins and hormones other than insulin. The HL-60 cell was able to grow and differentiate in this medium. The spontaneous differentiation along the granulocytic pathway after 72 hr, as assessed by the Nitro Blue tetrazolium test, increased by 400% compared to the serum supplemented medium, but the response to 1 microM retinoic acid was equal in the two media. Induction of monocytic differentiation by 0.16 microM phorbol-13-acetate-12-myristate, as determined by surface adherence after 24 hr, was lower in the absence than in the presence of serum. cAMP levels were elevated in response to (-)-isoproterenol. The EC50 was 0.36 +/- 0.01 microM (mean +/- SE, N = 3). The beta-adrenergic ligand 3H-CGP 12177 was specifically bound to 1 single class of binding sites (Kd: 0.15 +/- 0.04 nM, Bmax: 2220 +/- 150, mean +/- SE, N = 3). These data are comparable to our previously reported findings in serum supplemented medium. The present data show that HL-60 cells are able to grow and differentiate in the absence of serum proteins and hormones other than insulin. Under the present experimental conditions, these cells possessed functional beta-adrenergic receptors.

The effect of the plasticizers TBEP (tris-(2-butoxyethyl)-phosphate) and DEHP (di-(2-ethylhexyl)-phthalate) on beta-adrenergic ligand binding to alpha 1-acid glycoprotein and mononuclear leukocytes

The plasticizers tris-(2-butoxyethyl)-phosphate (TBEP) and di-(2-ethylhexyl)-phthalate (DEHP) and the beta-adrenergic receptor-blockers [3H]-(-)-dihydroalprenolol ([3H]-(-)-DHA) and [3H]-(-)-CGP 12177 were tested for their ability to interact with beta-adrenergic binding to alpha 1-acid glycoprotein (AAG) and mononuclear leukocytes (MNL). The IC50-values, obtained by displacement of [3H]-(-)-DHA bound to AAG, were 3.5 nM, 2 microM and 4 microM for TBEP, (-)-alprenolol and DEHP, respectively. (+/-)-CGP 12177 had virtually no effect on radioligand binding to AAG. The [3H]-(-)-CGP 12177 binding to MNL consisted of beta-adrenergic receptor binding (Kd = 210 pM) and non-saturable binding. [3H]-(-)-DHA was bound to two different classes of binding sites on MNL, the beta-adrenergic receptors (Kd = 440 pM) and a secondary class of binding sites (Kd = 64 nM). (+/-)-CGP 12177 displaced about 30% of [3H]-(-)-DHA from MNL with an IC50-value of 190 pm. (-)-ALP displaced about 85% of total bound radioligand and gave a biphasic displacement curve with IC50-values of 320 pM and 690 mM, respectively. TBEP displaced a considerable fraction of [3H]-(-)-CGP 12177 and [3H]-(-)-DHA bound to MNL beta-adrenergic receptors, whereas DEHP had no effect. In contrast, DEHP caused displacement of [3H]-(-)-DHA from the MNL low affinity sites, but was a markedly less potent displacer compared to TBEP. The present study shows that TBEP and DEHP interact with beta-adrenergic transport proteins, non-specific tissue binding sites and beta-adrenergic receptors coupled to adenylate cyclase. Plasticizers may thus affect the biology and pharmacology of the beta-adrenergic signal system.

Serum albumin stimulates serotonin uptake into human blood platelets

Active uptake of serotonin (5-hydroxytryptamine, 5-HT) into blood platelets from healthy donors exhibited a lower Vmax value in buffer media than in plasma. Also in plasma ultrafiltrate Vmax was reduced, but it returned to the level measured in plasma upon addition of human serum albumin (40 milligrams) containing fatty acids. Fatty-acid-free albumin was even more stimulatory and when added to platelets in a phosphate-buffered medium, it increased Vmax beyond the value observed in plasma. Km values calculated on the basis of unbound 5-HT were not affected by the media except for a slight decrease in ultrafiltrate as compared to plasma. The fraction of 5-HT (0.5 mumol/l) bound to 40 milligrams albumin was 17% with the preparation containing fatty acids and 22% with fatty-acid-free albumin, while total plasma proteins dissolved in phosphate buffer bound 24%. The uptake of 1 mumol/l 5-HT was enhanced by both albumin preparations already at 1 milligram and near-maximal effects occurred at 10 milligrams.

Kinetic and equilibrium studies of (-)125iodocyanopindolol binding to beta-adrenoceptors on human lymphocytes: evidence for the existence of two classes of binding sites

Saturation experiments were performed on intact human peripheral mononuclear leucocytes (MNL) and MNL membranes with (-)125Iodocyanopindolol (125ICYP) over a large concentration range (1.5-600 pmol/l). The corresponding Scatchard plots were curvilinear suggesting two saturable classes of binding sites: A high affinity binding site (Bmax1 = 1000 +/- 400 sites/cell, Kd1 = 2.1 +/- 0.9 pmol/l for intact MNL and Bmax1 = 550 +/- 190 sites/cell, Kd1 = 4.1 +/- 0.9 pmol/l for MNL membranes) and a low affinity binding site (Bmax2 = 9150 +/- 3590 binding sites/cell, Kd2 = 440 +/- 50 pmol/l for intact MNL and Bmax2 = 11560 +/- 4690 sites/cell, Kd2 = 410 +/- 70 pmol/l for MNL membranes). Dissociation of (-)125ICYP from MNL was biphasic consisting of a slow dissociating component (dissociation rate constant k-1 = (0.5 +/- 0.2) X 10(-3) min-1 for intact MNL and k-1 = (1.0 +/- 0.1) X 10(-3) min-1 for MNL membranes) and a fast dissociating component (k-2 = (80 +/- 20) X 10(-3) min-1 for intact MNL and k-2 = (60 +/- 10) X 10(-3) min-1 for MNL membranes). In dissociation experiments started after equilibration with various (-)125ICYP concentrations k-1 and k-2 were independent of the equilibrium concentration, whereas the percentual occupancy of the slow and the fast dissociating component varied and was similar to the estimated fractional occupancy of either binding site at the same (-)125ICYP concentrations in saturation experiments. The association rate constant was in the same order of magnitude for both binding sites. These results suggest two independent classes of binding sites for (-)125ICYP on MNL.

Modes of determining beta-adrenoceptor number in human mononuclear leucocytes

Inhibition of total 125I-ICYP binding to intact human mononuclear leucocytes at 32 degrees by propranolol and (+/-) CGP-12177 was biphasic. The high affinity component of 125I-ICYP binding, representing approximately 30% of total, was stereospecific, while the low affinity binding site was inhibited without stereospecificity. (-) Isoproterenol inhibited the high affinity component of 125I-ICYP binding only, with low affinity. By performing binding studies in intact cells at 4 degrees or in broken cell preparations at 37 degrees, the fraction of total 125I-ICYP binding representing specific binding was increased, and agonist affinity was high. Inhibition of 3H-CGP-12177 binding to intact cells at 32 degrees demonstrated a high fraction of specific binding and high agonist affinity. Computer-assisted analysis of total radioligand binding determined over a broad concentration range revealed two populations of saturable 125I-ICYP binding sites in intact cells as well as in broken cell preparations, while 3H-CGP-12177 binding demonstrated only one saturable binding site. The number of high affinity 125I-ICYP binding sites was comparable to the number of saturable 3H-CGP-12177 binding sites. Receptor numbers determined by analysis of total radioligand binding were comparable to receptor numbers determined by subtraction of non-specific binding, determined in the presence of a high concentration of competing ligand. Analysis of total radioligand binding was found to be a better procedure because it eliminates the use of an arbitrary concentration of unlabelled ligand and improves the accuracy of the assay.