Chemistry and Biology of Proteins - F. Haurowitz 1953

Albumins, globulins and other soluble proteins
Other animal albumins and globulins

Very few Proteins have been investigated in as much detail as Blood Plasma Proteins. One such protein is egg albumin, or albumin from chicken eggs. It was first obtained in crystalline form by Hofmeister [114], who used ammonium sulfate precipitation for this purpose. This method was subsequently improved by Hopkins [145] and Sörensen [146]. The aforementioned authors first removed globulin by half-saturation with ammonium sulfate, and then precipitated egg albumin by acidifying the filtrate. As already noted above, it is more convenient to use sodium sulfate instead of ammonium sulfate for precipitation [147]. To obtain crystalline egg albumin, protein from fresh eggs is mixed with an equal volume of a 36.7% sodium sulfate solution. The precipitated globulins are filtered off, and the filtrate is adjusted to pH 4.7 using dilute sulfuric acid; anhydrous sodium sulfate is then added until a slight opalescence appears. Upon standing, crystals of egg albumin form and settle to the bottom. The ease with which egg albumin can be obtained in crystalline form has made it one of the favorite subjects of research in Protein Chemistry. Some properties of egg albumin have already been mentioned in previous chapters; the Amino Acid Composition of this protein is given in Table 1.

Despite the fact that egg albumin forms exquisite crystals, it is not a homogeneous substance and consists, according to electrophoretic analysis, of at least two fractions [148, 149]. Egg albumin accounts for approximately 50% of the total protein content of chicken egg white; 15% is represented by conalbumin, a protein that remains in the filtrate after the precipitation of egg albumin crystals [150]. Conalbumin can be obtained directly from egg white by fractionation with dilute ethyl alcohol [151]. It turned out to be a glycoprotein with a Molecular Weight of 87,000 and an isoelectric point at pH 6.1; with iron salts, conalbumin forms a red complex (see p. 239). Another protein, designated as egg globulin, can also be isolated from egg white, though little is known about its properties [152].

Egg white also contains Avidin, a basic protein. This protein is responsible for the physiological disorders in animals observed when consuming raw egg white. Avidin has been obtained in crystalline form [153]. It is a basic glycoprotein with an isoelectric point near pH 10 [154]. Avidin forms compounds with biotin, and the aforementioned disorders are likely caused by the resulting deficiency of free biotin [155]. The avidin–biotin complex is very similar in its properties to Lysozyme, and it is possible that the two compounds are identical [156, 157].

The main milk protein [158] is casein, which belongs to the group of Phosphoproteins (see p. 237). It precipitates upon the acidification of milk. The filtrate (whey) contains lactalbumin and lactoglobulin. The molecular weight of lactalbumin is 17,400 [159]. If the albumin fraction precipitated with ammonium sulfate is dissolved in Water and dialyzed under slightly acidic conditions, crystals of so-called ß-lactoglobulin precipitate [160]. Because this protein can be easily obtained in significant quantities, it has been the subject of numerous studies. Information regarding its molecular weight, as well as the size of its molecules, is given in Tables 4 and 5.

Proteins of the albumin and globulin types can be isolated not only from Body Fluids, but also from various Organs. In such cases, it is advisable to first remove blood from the organs by perfusing them with an isotonic sodium chloride solution. After mincing and grinding, the blood-free organ is extracted with sodium chloride solutions or Buffer solutions. Proteins obtained in this manner from Muscle are described in the section "Muscle Proteins." Many of the proteins isolated from organs exhibit enzymatic activity; they will be discussed in Chapter XII. Protein Hormones isolated from Endocrine glands are discussed in Chapter XIV. Organ extracts contain proteins belonging mainly to globulins, i.e., those proteins that precipitate upon the acidification of solutions, by salting out, or by dialysis against distilled water. Proteins of the albumin type are rarely found in such extracts. For instance, 51.5% of the total protein in the eye lens consists of globulin, and only 0.5% is albumin; the remaining 48% is accounted for by an insoluble protein, scleroprotein. Two proteins, designated as a- and ß-crystallins, have been isolated from the lens globulin fraction [161].