Water-soluble molecules, insulin

All hydrophilic (water-soluble) molecules (which cannot diffuse across the hydrophobic interior of the lipid bilayer) bind to receptors in the plasma membrane. There are two subclasses of hydrophilic hormones (1) peptide hormones such as insulin and glucagon and (2) small charged molecules, often biogenic amines, such as epinephrine (adrenalin) and histamine that are derived from amino acids and function as hormones and neurotransmitters (see Topic N3). 

Water-soluble peptide and amine hormones (insulin and epinephrine, for example) act extracellularly by binding to cell surface receptors that span the plasma membrane (Fig. 23-4). When the hormone binds to its extracellular domain, the receptor undergoes a conformational change analogous to that produced in an allosteric enzyme by binding of an effector molecule. The conformational change triggers the downstream effects of the hormone. 

An experimental complication is the difficulty in effecting molecular interaction between the components. The usual technique for preparing lipid-protein phases in an aqueous environment is to use components of opposite charge. This in turn means that the lipid should be added to the protein in order to obtain a homogeneous complex since a complex separates when a certain critical hydrophobicity is reached. If the precipitate is prepared in the opposite way, the composition of the complex can vary since initially the protein molecule can take up as many lipid molecules as its net charge, and this number can decrease successively with reduction in available lipid molecules. It is thus not possible to prepare lipid— protein—water mixtures, as in the case of other ternary systems, and to wait for equilibrium. Systems were prepared that consisted of lecithin-cardiolipin (L/CL) mixtures with (a) a hydrophobic protein, insulin, and with (b) a protein with high water solubility, bovine serum albumin (BSA). 

Long-acting insulin analogues. The more extensive alteration of amino acids in insulin glargine changes the electric charge of the molecule. At pH 4 of the injectate, it is dissolved however, at the pH of tissue it is poorly water-soluble and precipitates. Resolubilization and diffusion into the bloodstream take about one day. 

The insulin molecule contains six amino acid residues that can carry a positive charge and 10 that can carry a negative charge. The net charge is zero at pH 5.5 (Brange, 1987). The solubility of insulin is dependent on the pH of the solvent, being practically insoluble in water at pH 5.4 but easily soluble at pH less than 4. In alkaline media, solubility is dependent on the concentration of zinc ions and the species of insulin origin (Schlichtkrull, 1958). 

Adrenocorticotropic hormone (ACTH), insulin, and parathyroid hormone (PTH) are examples of polypeptide or protein hormones. They are generally water soluble and circulate freely in plasma as the whole molecule or as active or inactive fragments. The half-life of these hormones in plasma is quite short (10 to 30 minutes or less), and wide... 

The graph in Fig. 2 can be used to address the most fundamental question regarding insulin iontophoresis—is it theoretically possible, under the most favorable conditions, to deliver the required dose Based on the discussion in Section 2.1, the needs are a basal delivery rate of 1-2 units per hour coupled with a bolus of up to 20 units over about a half-hour. Figure 2 indicates that a delivery rate of 40 units per hour could be achieved with a 1 mM solution of insulin, which is equivalent to about 4mg/ml or 100 units/ml. Regular (currently marketed human, pork, or beef) insulin has a water solubility that exceeds this value. Thus, it is theoretically possible to iontophorese insulin at the required rate. However, an idealized model has been used to reach this conclusion. Specifically, it was assumed that insulin exists as an ideal solution (with a MW of 5800), that the mobility of insulin is independent of pH and has a value close to its maximum value, and that the molecule is not degraded on its way through the skin. For regular insulins, these assumptions are not true. In the next section, the physicochemical properties of insulin that impact its deliverability by iontophoresis are described. 

ABNP is soluble in dimethylformamide (DMF) but insoluble directly in aqueous solution. Insulin labeling was done in DMF water at a ratio of 9 1. For molecules not soluble in organic solvent, such as proteins, the trifunctional first may be dissolved in DMF and a small aliquot added to an aqueous reaction medium. The nitrophenyl ester reactive group can be coupled to amine groups at alkaline pFI (7-9) and in buffers containing no extraneous amines (avoid Tris). Unfortunately, ABNP is not commercially available at the time of this writing. 

A protein called keratin is insoluble in water but binds to other keratin molecules to form hard structures. Proteins like keratin are known as structural proteins or fibrous proteins. Keratin is the main component in hair and fingernails. A different protein called insulin is soluble in water and achieves a certain shape as an individual molecule. Proteins like insulin are known as globular proteins. Insulin in the bloodstream regulates glucose metabolism. 

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