Solubility proteolytic modification

Proteolytic modification has special importance for the improvement of solubility of proteins. This effect becomes significant even after very limited proteolysis. Hydrolysis of casein to DH of 2 and 6.7% with Staphylococcus aureus V8 protease increased the isoelectric solubility to 25 and 50%, respectively (Chobert et al., 1988a). However, it should be noted that the solubility profiles were not identical, due to a shift of the isoelectric point of the modified proteins. Solubility of a protein hydrolysate depends on the enzyme used (Adler-Nissen, 1986a). Protamex (a Bacillus proteinase complex) hydrolysates of sodium caseinate (DH 9 and 15%) displayed 85-90% solubility between pH 4 and 5 (Slattery and FitzGerald, 1998). 

Hydrophilic hormones and other water-soluble signaling substances have a variety of biosynthetic pathways. Amino acid derivatives arise in special metabolic pathways (see p. 352) or through post-translational modification (see p. 374). Proteohormones, like all proteins, result from translation in the ribosome (see p. 250). Small peptide hormones and neuropeptides, most of which only consist of 3-30 amino acids, are released from precursor proteins by proteolytic degradation. 

As a result of the contact of blood with none-ndothelial surfaces, several humoral and cellular systems can be activated. Exposure of blood proteins and cells to blood contacting medical devices can activate plasma proteolytic systems (coagulation (blood clotting system), fibrinolysis (process by which clot is broken down), complement cascade (a system of soluble proteins involved in microbiocidal activity and the release of inflammatory components), Kallekrein-kinin and contact systems) and at least three cellular elements (leukocytes, endothelial cells, and platelets). Contrary to the normal situations whereby these mechanisms are localized and intended to promote wound healing, activation of these systems by medical devices can result in nonlocalized systemic reactions. The preclinical and clinical assessments of hemocompatibility are designed to minimize modification of these systems. 

Of the many actual and potential uses of enzymes for chemical modification and improvement of proteins, that of hydrolysis of proteins is the most widely used. Hydrolysis involves the action of selected proteolytic enzymes to split specific peptide bonds in a protein. Along with a decrease in size of the protein there are changes in the solubility and functional properties of the product. Some of the uses of proteolytic enzymes in protein modification are shown in Table I. 

Modification of fish proteins by proteolytic enzymes to increase their solubilities illustrates a variety of techniques and approaches. Basically, three general enzymic methods have been used to prepare fish proteins or hydrolysates with altered solubilities and other functionalities. These methods include (a) the enzymic solubilization of fish protein concentrate prepared by hot solvent extraction of fish, (b) the enzymic modification of myofibrillar proteins extracted from fish with 0.6M NaCl, and (c) the proteolysis of whole fish to prepare biological fish protein concentrate (FPC). 

There have been a limited number of studies on the effects of enzymic modification of protein concentrates on functional properties other than solubility. Studies on functional properties, as modified by enzymic treatments, emphasize foam formation and emulsifying characteristics of the hydrolysates. Treatment of chicken egg albumen alters the functional properties of the egg proteins in terms of foam volume and stability and the behavior of the proteins in angel food cakes (25). Various proteolytic enzymes were used to degrade the egg albumen partially. However, proteolytic enzyme inhibitors indigenous to the egg proteins repressed hydrolysis of the egg proteins compared with casein. 

The function of these additional chemical groups is not well understood. In some cases, they serve as molecular switches, activating the protein only when it is needed. Glycosolation is the most prevalent modification in eukaryotic cells, and it appears to increase the protein s solubility and decrease its susceptibility to proteolytic attack. Lipid attachment to proteins is usually seen when the protein s function requires its presence near a membrane surface within the cell. The polar portion of the lipid is attached covalently to the protein while the hydrophobic tail of the lipid is imbedded in the membrane. The lipid group thus serves as an anchor for the soluble protein. Finally, many of the enzymes... 

There are hundreds of types of post-translational modification (proteolytic processing, addition of chemical groups, side chains) of proteins, often determining protein immunogenicity, function, solubility, and so on. This area was largely unexplored. 

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