Elastolytic enzymes

The literature relating to collagenases and elastases has recently been the subject of a comprehensive review by Ines Mandl (1961) and no purpose would be served by attempting a similar description here. It is proposed instead to summarize briefly some of the salient features arising from the work of the last decade and to draw attention to a number of recent researches that have led to advances in the study of elastolysis by enzymes. 

With elastin preparations as described above, the classic proteolytic enzymes of the pancreas—trypsin and the chymotrypsins—have little activity and do not produce dissolution of the fibers. In the early literature there are many reports of the dissolution of elastin libers by pan-creatin or by impure preparations of trypsin, but it was not until the work of Balo and Banga (1949) that the existence of a separate elastolytic enzyme in the pancreas was recognized. Soon after this Banga (1952) obtained a purified crystalline enzyme which was thought to be substantially free from nonelastolytic components of the pancreatic system of enzymes. 

The pancreas of all mammals so far investigated contain an elastase with similar enzymatic reactions (Lewis et al., 1956 Marrama et al., 1959), but immunological differences have been observed between pancreatic elastases from different species (Moon and Mclvor, 1960). Elastase is secreted in the pancreatic juice as an inactive zymogen, proelastase (Grant and Robbins, 1955 Lamy and Lansing, 1961) which, like other pancreatic enzymes, is activated by trypsin or enterokinase. 

Although it is now generally agreed that pure preparations of trypsin and chymotrypsin do not digest elastin there have been frequent reports in the early literature that pepsin is capable of dissolving elastic fibers. This was repeated recently by Fisher et al. (1960) who observed moderate elastolytic activity by three different crystalline pepsin preparations at pH 1.2. The action of pepsin was estimated to be approximately one-eighth 

Elastase activity is not a universal property of proteolytic sulfhydryl-activated enzymes. There are abundant reports in the literature describing the disappearance of elastic fibers in vivo preceding the repair of damaged tissues, but there is no evidence as to how this is brought about. The tissue cathepsins, most of which are SH-activated, have received little systematic study, but Thomas and Partridge (1960) reported that cathepsins extracted from kidney and spleen by the method of De La Haba et al. (1955) did not digest elastin either in the presence or the absence of cysteine. 

---

The kinetics of the enzymatic dissolution of elastin are very complex and the reaction of elastin fibers with elastolytic enzymes is commonly characterized by a lag phase or a markedly sigmoid time course (cf. Hall and Czerkawski, 1959 Naughton et al., 1960). The length of the slow initial phase is influenced by the enzyme-substrate ratio and the source and method of preparation of the elastin used in the assay the course of the reaction with different enzymes is also influenced by the presence or absence of salts, reducing agents, and many other substances. As a result... 

It has already been remarked that most enzymes with elastolytic activity have proved to be proteinases with a wide peptide-bond specificity. Thus papain, bromelin, and ficin have a similarly broad hydrolytic action and are all active elastolytic enzymes. Sanger et al. (1955) found that the oxidized A chain of insulin was hydrolyzed in a variety of positions by either crude papain or activated mercuripapain. There were five major... 

The turnover rate of mature elastin in healthy persons is relatively low. Insoluble elastin in healthy elastic tissue is usually stable and subjected to minimal proteolytic degradation. In several clinical conditions (e.g., emphysema, advanced atherosclerosis, pancreatitis), increased degradation of fragmentation of elastic fibers may play a significant role. The interaction between insoluble elastin and soluble elastolytic enzymes, and the regulation of these enzymes, may shed light on certain cardiovascular diseases, in view of the role of elastin in arterial dynamics. 

A second elastolytic enzyme (Af, 21,900) has been isolated from porcine pancreas. It shows higher activity than chymotrypsin in the hydrolysis of acetyltyrosine ester, which is used routinely to assay chymotrypsin. Another E,-like enzyme, a-lytic proteinase (Af, 19,900, 198 amino acids) has been isolated from the soil bacterium Myxobacter 495. This enzyme is remarkably similar to pancreatic E. both in structure (41 % homology, sequence in the active center Gly-Asp-Ser-Gly, 3 homologous disulfide bridges) and substrate specificity. Another E. (Af, 22,300) has been isolated from Pseudomonas aeruginosa. 

There are two t3 pes of mechanism that could account for an elastolytic reaction of high specificity. First, the enzyme could attack the crosslinks in elastin without rupture of peptide bonds. This kind of mechanism has been suggested by several authors, and raises the possibility of elastoly-sis by a nonproteolytic enzyme. Alternatively the specificity of the reaction may be due to the presence in elastin of an amino acid sequence of some length which is not present in other proteins. In this case the enzyme would differ from most other proteolytic enzymes which seem to display specificity for short sequences or a single type of peptide bond. 

Some protoberberine and structurally related alkaloids were tested for inhibitory activity on porcine pancreatic elastase (PPE) and human sputum elastase (HSE). Berberine chloride significantly inhibited the elastolytic activity of both enzymes, but tetrahydroberberine had no effect. It appears that the quaternary nitrogen atom of these alkaloids plays an important role in the inhibition of elastolytic activity. The amidolytic activity of the elastases was not affected by any of the test alkaloids [240]. 

---