Nucleic acid hydrolases

Several hydrolases—particularly ribo-nuclease (RNAse) and deoxyribonuclease (DNAse)—break down the nucleic acids contained in food. 

Since lysosomes are involved in digesting a whole range of biological material, exemplified by the destruction of a whole bacterium with all its different types of macromolecules, it is not surprising to find that a large number of different hydrolases reside in lysosomes. These enzymes catalyze the breakdown of nucleic acids, proteins, cell wall carbohydrates, and phospholipid membranes (see Table 1.1). 

Many of the nucleic acid hydrolases occur in isoenzymic forms in mammalian tissues. Some of these isoenzymes are glycoenzymes, and information on these is contained in Table I. Ribonuclease B has been studied in the greatest detail. Following the initial observation that the ribonuclease from bovine pancreas occurs in isoenzymic forms,45 it was found that one form (ribonuclease B) contains an appreciable... 

Many cells are susceptible to the appreciable shearing forces that arise on repeated freezing and thawing, or to hypotonic buffers which cause cells to swell up, and in certain cases to lyse this is particularly the case for cells in soft plant and animal tissue. Such treatments only rarely lead to complete cell lysis, the exceptions to this being erythrocytes and reticulocytes which are lysed quantitatively under hypotonic conditions. Non-mechanical homogenisation is of particular relevance to cells like yeast which are refractory to other procedures. One of the simplest procedures for yeast, which can certainly not be described as gentle, is toluene-induced autolysis. This is carried out at room temperature and leads to permeabilisation of the cell walls this causes various hydrolases to be activated causing breakdown not only of the cell structure, but also (undesirably) of many sensitive proteins and nucleic acids in the cell. Consequently, this process is mainly of historical interest. 

Hotelier T, Renault L, Cousin X, Negre V, Marchot P, Chaton-net A. ESTHER, the database of the alpha/beta-hydrolase fold superfamily of proteins. Nucleic Acids Res. 2004 32 D145-147. 

Robins, M. J., Wnuk, S. F., Mullah, K. B. and Dailey, N. K. (1994) Nucleic Acid related compounds. 80. Synthesis of 5 -5-(alkyl and aryl )-5 -(l uoro-5 -thioadenosi ncs with xenon difluoride or (diethylamido)sulfur trifluoride, hydrolysis in aqueous buffer, and inhibition of S-adenosyl-L-homocysteine hydrolase by derived adenosine 5 -aldehyde species. J. Org. Chem., 59, 544-555. 

Excessive accumulation of proteins, nucleic acids, carbohydrates, and lipids can result from deficiency of one or more lysosomal hydrolases. Lysosomal storage diseases are classified by the stored material. Accumulation of gly-cosaminoglycans results in mucopolysaccharidoses. Common causes of this disorder include Hunter syndrome, Hurler syndrome, and Sanfihppo syndrome. Sanfdippo syndrome is inherited in an autosomal recessive pattern and clinically evident by profound mental retardation, lack of normal developmental milestones, and significant language delay. Sanfdippo syndrome results in an excess of heparan sulfate and can be caused by a variety of enzyme deficiencies. 

The excision of nucleic acid bases by hydrolases can be both a repair mechanism for the host and an offensive weapon by competitors. There is a whole array of DNA repair glycosylases, some of which are merely hydrolases, others of which have an associated lyase activity.Catalysing a similar reaction are the proteins, formed in the seeds of many plants, which hydrolyse a ribosyl-adenine bond in the sequence GAGA loop region of the 28S RNA, thus shutting down protein synthesis.No direct determination of reaction stereochemistry has been carried out for either class of enzyme, nor is an X-ray structure of an enzyme-substrate-analogue complex available, but the crystal structures so far available suggest inversion. 

The swelling ability of these naturally occurring materials provides unusual properties and appreciable surface area for adsorption of organic molecules [15, 30], as was mentioned in Section 2.2.1.1. For the last 25 years different research groups have studied the immobilization of different biomolecules, including enzymes, proteins, nucleic acids [31] as well as cells [32-35] in layered materials. More specifically, various enzymes have been immobilized on clays, including hydrolases and oxidoreductases (see Table 2.1). 

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