Globulins, plasma solubility

Not many purified proteins have been found to reduce chromate at physiological pH. Egg albumin and human plasma proteins were unreactive toward Cr(VI) at pH 6.4 and pH 7.35 . Purified cytochrome c did not react with CrOj" and bovine serum albumin , human serum albumin, serum globulin and soluble dermis protein did not react... 

The diversity in primary, secondary, tertiary, and quaternary stmctures of proteins means that few generalisations can be made concerning their chemical properties. Some fulfil stmctural roles, such as the collagens (found in bone) and keratin (found in claws and beaks), and are insoluble in all solvents. Others, such as albumins or globulins of plasma, are very soluble in water. Still others, which form part of membranes of cells, are partly hydrophilic ( water-loving , hence water-soluble) and partly lipophilic ( lipid-loving , hence fat-soluble). 

Some 100 different proteins occur in human blood plasma. Based on their behavior during electrophoresis (see below), they are broadly divided into five fractions albumins and ai-, tt2-, P- and y-globulins. Historically, the distinction between the albumins and globulins was based on differences in the proteins solubility -albumins are soluble in pure water, whereas globulins only dissolve in the presence of salts. 

The table also lists important globulins in blood plasma, with their mass and function. The a- and p-globulins are involved in the transport of lipids (lipoproteins see p. 278), hormones, vitamins, and metal ions. In addition, they provide coagulation factors, protease inhibitors, and the proteins of the complement system (see p. 298). Soluble antibodies (immunoglobulins see p. 300) make up the y-globulin fraction. 

Antibodies (immunoglobulins) constitute the gamma globulin part of the blood proteins. They are soluble proteins secreted by the plasma offspring (clones) of activated B-cells. 

When the red blood cells are removed from whole blood, and the resulting plasma is allowed to clot, the fibrinogen may be removed to yield serum. Serum contains a variety of proteins, some of which are called globulins because their solubility properties are different from the other serum proteins. Antibodies are a subclass of serum globulins that possess selective binding properties. Antibodies are also called immunoglobulins (Ig). 

Osmotic balance across the plasma membrane is regulated by the concentration of proteins (albumin more than globulins). Relative dehydration would increase plasma oncotic pressure and capillary absorption of water and water-soluble molecules. Phenol is soluble in water, and its absorption rate could be accelerated by the slightest dehydration. 

In 1932, Race discovered that plasma albumin after precipitation with TCA was soluble in alcohol and, further, that horse, human, sheep, and ox albumin were completely soluble, while their globulins were totally insoluble, in a solution containing 20 g TCA, 80 ml acetone, and 15 ml water. At the same time Race (Rl) devised a method for determining the albumin in plasma by first extracting it from TCA-precipitated proteins with 80% aqueous acetone containing 2-3 ml 20% TCA. Albumin nitrogen was then determined in the extract. With this acid-acetone fractionation technique, plasma from patients with rheumatoid arthritis gave lower albumin levels than those found by Howe s method, and, indeed, these were probably the first really accurate albumin assays on blood to be reported. 

Heparinoids and mucopolysaccharides react with, and modify, many of the plasma proteins. Heparin combines with fibrinogen, globulins and albumin. As judged by electrophoresis and various types of analysis and staining, the particular plasma protein components with which heparin combines are dependent upon the concentration of protein, concentration of heparin, pH value, and salts present. This explains the somewhat contradictory statements in literature about combinations of heparin with plasma proteins. The combination may result in change of solubility of the protein and reverse protein tests . Heparin can modify the murexide reaction for calcium in serum by affecting the calcium-protein-heparin complex. Many heparinoids... 

On the whole, however, the usual relation— absorption of heat on solution and increase of solubility with temperature— is found much more commonly in protein systems, especially at low ionic strengths. Differ t proteins often differ markedly in heat of solution. The classical method of preparation of edestin involved its crystallization by the gradual cooling of a very warm salt solution of the protein. Marked differential effects of temperature on solubility have been observed for different components among the globulins of normal plasma, notably for y- obulin, and fibrinogen, and have been used already in certain procedures, especially for the purification of fibrinogen (Part VI). 

The presence of lipids and steroids in close association with certain plasma protein fractions has often been observed. An excellent brief account of some of this earlier work has already been givmi by Chargaff (28) in an earlier volume of this series. Macbeboeuf (134) in 1929 obtained a water-soluble (or salt-soluble) lipoprotein from horse serum by precipitating most of the globulin with half-saturated (NH4),S04, and acidifying the filtrate to pH 3.8. The precipitate so separated was... 

Factors Affecting Stability to Heat Plasma and serum are notable for their capacity to hold in stable aqueous solution many substances whose solubility in pure water or salt solutions is extremely low. The remarkable capacity of certain globulin fractions— notably those from Fractions III-O and IV-1, as separated by the low temperature-ethanol technique— to bind cholesterol, phos-phatides, and other types of lipid material, in stable, water-soluble combination has already been described in the preceding section. Serum albumin, however, displays equally remarkable afiSnities for a somewhat different and extremely diverse group of substances, including many types of acid and basic dyes, the ions of many organic acids containing hydrocarbon residues, certain quinone derivatives, and other substances. 

Makino et al. examined the effect of plasma proteins on the degradation characteristics of PLAioo microcapsules and showed that the degradation rate in an aqueous medium was accelerated by the addition of albumin, y-globulins and fibrinogen [146]. This was interpreted by the electric potential distribution and the increased solubility of PLAioo in the presence of plasma proteins. 

Lipids, water-insoluble compounds, do not enter the circulation as such but as components of soluble lipoprotein complexes. Apart from FFA, which are bound to plasma albumins, the other plasma lipids (free and esterified cholesterol, phospholipids and triglycerides) are bound to proteins which migrate electrophoretically with ai- and i-globulin fractions of plasma, and they are therefore termed a- and -lipoproteins. 

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