Plasma proteinase inhibitors.

Plasma proteinase inhibitors account for about 10% of the total protein in plasma. They provide one mechanism for the control of proteinase activity, thus regulating many important biological reactions such as blood coagulation. Most plasma inhibitors are specific for one or a few related proteinases and control a particular biological event or pathway. Two other inhibitors, the alpha 2-macroglobulin (alpha 2M) and the alpha 1-proteinase (alpha 1PI = alpha 1-antitrypsin) have a broader specificity. The role of alpha 1PI, although theoretically able to inhibit a large number of enzymes, is the inhibition of leukocytic elastase. This function is particularly important in the lung where elastase may be released from neutrophils particularly in smokers. alpha 2-Macroglobulin reacts with a large number of very different proteinases by a mechanism quite different from those of the other inhibitors. The physiological role of this inhibitor is not clearly understood although it may act as a "back-up" inhibitor when levels of other inhibitors are low or if no specific inhibitor is available.