Pepsin, isoelectric point

Just as individual amino acids have isoelectric points, proteins have an overall p/ because of the acidic or basic amino acids they may contain. The enzyme lysozyme, for instance, has a preponderance of basic amino acids and thus has a high isoelectric point (p/= 11.0). Pepsin, however, has a preponderance of acidic amino acids and a low- isoelectric point pi 1.0). Not surprisingly, the solubilities and properties of proteins with different pi s are strongly affected by the pH of the medium. Solubility- is usually lowest at the isoelectric point, where the protein has no net charge, and is higher both above and below the pi, where the protein is charged. 

It is well known in biochemistry that many proteins, especially enzymes but also transport proteins such as albumin, show high enantioselectivities in their interactions with small chiral molecules. It is possible to bind proteins to silica and to obtain a valuable class of CSPs that is mainly suited for the separation of chiral drugs. Several protein phases are commercially available albumins, a-acid glycoprotein, ovomucoid, avidin, cellobiohydrolase I, and pepsin. They differ in their chromatographic and enantioselective properties which is not a surprise because their biological functions and their size, shape or isoelectric point are not identical at all. 

White powder. Freely sol in water. Partly precipitated at the isoelectric point (pH 4.65-4.80). Appreciably soluble in 60 to 70% alcohol or acetone. Almost completely precipitat -ed in 2.5% trichloroacetic acid soln. Also precipitated from dilute soln by 20% sulfosalicylic acid and by 5% lead acetate soln. Solns are stable to heat. An aq soln buffered to pH 7.5 may be put in a boiling water bath for at least 120 minutes. In 0.10 molar HC1 an 0.2% soln retains its biological potency when kept at 100° for 60 min, but in 0.10 molar NaOH the activity is lost within 30 min. At 60° at 0.2% soln at pH 10.8 maintains its potency for 60 min. In general the substance is more stable in acid soln. Pure ACTH has 150 to 200 potency units per mg. The hiological activity is not destroyed by digestion with pepsin, and hydrolysis products have the biological activity of ACTH. One U.S.P. unit or one international unit or one Armour unit or one potency unit denotes the same activity. 

White powder. Isoelectric point 4.6 (sheep), 7.45 (pig). Soluble in water. Chymotrypsin, trypsin and pepsin destroy the gonadotrop(h)ic action of LH. Picrolonic, flavianic. picric, and trichloroacetic acids precipitate LH with retention of its physiological activity. therap cat Gonadotropic hormone. 

White or yellowish-white translucent scales or granules, or an amorphous slightly hygroscopic powder, or spongy masses. It has a slightly acid or saline caste. (o] f —64.5 (water pH 4.6). Isoelectric point water with more or less opalescence. Practically insol in ale, chloroform, ether. Very stable to acid. The activity of pepsin in solns is destroyed by heating above 70, or by alkalies dry pepsin is not injured by heating to 100. Incompat. Ale, tannin, alkaline substances and salts of heavy metals... 

Covalent linking of enzyme on fused silica capillaries was also demonstrated by Stigter [119]. Pepsin was covalently immobilized on dextran-modified capillaries as illustrated in Scheme 10.12. The applicability of pepsin-immobilized microreactor was tested with a number of proteins varying in molecular weight, isoelectric point, and sample composition. This open tubular microreactor demonstrated a flow-dependent digestion as represented by native hemoglobin. Complete digestion of... 

Active pepsin was crystallized by Northrop. It is a protein with a molecular weight of 34,500, whose isoelectric point is unusually low (below 1). A phosphoric acid residue in the molecule, although dispensable for the catalytic action, is responsible for the very low pi. Pepsin has optimal activity at the acidity of the gastric juice (for pH optimum see Table XI). 

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