Protein Degradation in health and disease.

Plasma levels o f a n t i t h r o m b i n I I I , «2-macrog lobul in and inter -at ryps in i n h i b i t o r , as wel l as those o f various c l o t t ing , complement and other plasma factors, were s igni f i cant ly decreased in 18 patients suf fer ing f r o m hyperdynamic septic shock. A s imi lar statistically signif icant reduct ion o f the concentrat ions o f several plasma factors ( p r o t h r o m b i n and a n t i t h r o m b i n I I I , p lasminogen and «2-plasmin i n h i b i t o r , complement factor C3 and c l o t t ing factor X I I I ) was observed in exper imental endotoxaemia. I n this model the reduct ion in the plasma levels o f these factors was considerably d imin ished by the intravenous in ject ion o f a granulocyt ic elastase-cathepsin G i n h i b i t o r o f lower molecular weight f r o m soybeans. The results o f bo th studies indicate that consumpt ion o f plasma factors in the course o f Gram-negat ive sepsis proceeds not on ly via the classical routes (by ac t i va t ion o f the c lo t t ing , f i b r ino l y t i c and complement cascades by system-specific proteinases such as thrombok inase or the plasminogen act ivator ) bu t also to an appreciable degree by unspecific degradat ion o f plasma factors by neutra l proteinases such as elastase and cathepsin G . The endotox ininduced release o f b o t h sorts o f proteinases, the system-specific ones and the unspecif ic lysosomal proteinases f r o m leucocytes and other cells, is l ike ly to be main ly responsible for the c onsumpt i on o f a n t i t h r o m b i n I I I and c*2-macroglobulin via complex f o r m a t i o n ( f o l l owed by e l im ina t i on o f the complexes) and the increased turnover o f the in t e r -a t r yps in i nh ib i t o r as observed in the c l in ica l study. The therapeutic use o f an exogenous elastase-cathepsin G i n h i b i t o r in the exper imental mode l was s t imula ted by the observat ion that h u m a n mucous secretions conta in an acid-stable i n h i b i t o r o f the neutra l granulocyt ic proteinases, called H U S I I o r ant i leucoprote inase. Th is i nh ib i t o r protects mucous membranes 351 © Excerpta Medica 1980 Protein degradation in health and disease (Ciba Foundation Symposium 75) ρ 351-379 352 Η. 1 RITZ ET A I . and soluble proteins against proteo ly t ic attack by leucocytic proteinases released in the course o f a local i n f l ammato ry response. P re l iminary results indicate that HUS1I , wh i ch is produced by the epithel ia l cells o f mucous membranes, does not belong to any k n o w n st ructura l type o f acid-stable proteinase i nh ib i t o r . The search for other candidates suitable for medicat ion in humans led to the discovery o f a potent elastase-cathepsin G inh ib i t o r , called eg l in, in the leech Hirudo medicinalis. This acid-stable i n h i b i t o r w i t h a molecular weight close to 8100 has an unusual s t ructura l proper ty in that the structure o f the molecule is not stabil ized by any d isulphide bridge. The course o f a disease l ike septicaemia or septic shock is o f ten complicated by severe pathobiochemical processes in the c i rcu la t ion . These are, for example, disseminated intravascular coagulat ion (D IC ) caused by act ivat ion o f the c lo t t ing and f ibr ino ly t i c cascades, and anaphylactic responses induced by act ivat ion o f the complement system (Hami l t on et al 1978, MüllerBerghaus et al 1976, Garner et al 1974, McCabe 1973). These act ivat ion reactions may be triggered by endotoxins — that is, l ipopolysaccharides f r o m Gram-negative bacteria (Jeljaszewicz & Waldström 1978, Urbaschek et al 1975). Endotox ins can damage biological membranes and thus induce the release o f constituents, so-called mediators o f i n f l ammat i on , inc lud ing lysosomal enzymes, f r om various body cells (Weissmann 1974, Urbaschek et al 1975, M y r v o l d 1976, Mova t 1979). These enzymes normal ly exhibit their physiological func t ion , namely degrading phagocytosed mater ia l , inside the cell (K lebanof f & Clark 1978). I f released into the c i rcu la t ion , however, they may enhance the in f l ammatory response by several routes. System-specific proteinases such as the plasminogen activator and thrombokinase activate the b lood systems (see Fig. 1) by proenzyme — enzyme conversion — that is, by specific proteolyt ic cleavages. These enzymes are responsible, therefore, for the 'classical' or specific consumpt ion o f factors o f these systems, inc luding the inhib i tors o f c lo t t ing ( A T I I I ) , ka l l ikre in ( C I I N A ) , f ibrinolysis (azPI) and complement ( C I I N A ) factors (cf. Fig. 2 and Table 1). E l im ina t i on o f both the proteinases and inhib i tors o f the b lood systems also proceeds specifically by the f o rmat ion o f enzymeinh ib i t o r complexes that are phagocytosed by cells o f the ret iculoendothel ial system (RES) (Ohlsson 1974, Ohlsson & Laure l l 1976, Ohlsson 1978). More recently it became evident f r om studies in vitro and in vivo that endotoxininduced consumpt ion o f plasma proteins might also be due to a significant degree to unspecific degradation by leucocytic proteinases, especially an elastase (cf. Figs. 1 and 2) (Schmidt et al 1974, Haschen 1975, Egbr ing et al 1977, Egbr ing & Havemann 1978, Schiessler et al 1978a, Aasen PROTEINASE INHIBITORS IN I N F L A M M A T I O N 353 Proteinases released from cells can cause: Activation 1 Blood systems such as: clotting, fibrinolysis, complement, kallikrein 1 S p e c i f i c Degradation I Blood system factors, other blood proteins, proteinase inhibitors I U n s p e c i f i c c o n s u m p t i o n F I G . 1. Effects of proteinases released from blood and tissue cells during inflammation on blood systems and plasma factors. & Ohlsson 1978). I n such unspecific consumpt ion reactions — wh ich , i n contrast to the specific consumpt ion , are not l imited to the factors o f the b l o o d systems (cf. F ig . 1) — the biological act iv i ty o f the plasma proteins is irreversibly destroyed by proteo lyt ic degradation. The l iberated leucocytic proteinases are e l iminated, however, i n a specific manner by the f o rmat i on o f complexes w i th proteinase inh ib i to rs such as c ^ M , ct\A and cnAC (see Fig . 2 and Table 2) and the phagocytosis o f the complexes by the RES (Ohlsson & Laure l l 1976, Debanne et al 1976). Consumption reactions Specific activation Unspecific degradation e.g. thrombokinase, plasminogen activator e.g. PMN elastase, cathepsins G, B, D 1 Blood systems I Blood proteins e.g. AT II Plasma proteinase inhibitors 1 cx2P\, C1 INa e.g. αλ A, a i x AC, a 2M 1 ion (RES) F I G . 2. Consumption of plasma factors during inflammation. System-specific proteinases such as thrombokinase activate the blood system factors which are inhibited thereafter by complex formation with antithrombin I I I (AT I I I ) , a2-plasmin inhibitor (a2PI) and CI inactivator (CI INA) , respectively. Proteinases liberated from blood and tissue cells like granulocytic (PMN) elastase can degrade plasma factors unspecifically before being inhibited by complex formation with cYj-antitrypsin (αϊA) , aj-antichymotrypsin (ajAC) and c*2-macroglobulin (c*2M), respecti­ vely. The enzyme-inhibitor complexes are eliminated from the circulation by cells of the reticu­ loendothelial system (RES). 354 Η. FRITZ ET ΑΙ .