A sufficient explanation for the observations that HAE is a dominantly transmitted disease and that the hemizygotes have levels of the normal protein of only in the region of 15%-20% of normal can be given by proposing that a substantial proportion of the catabolism of C1(-)-esterase inhibitor involves the prior formation of a complex with one of the enzymes with which the inhibitor reacts. This part of the catabolism will be largely independent of inhibitor concentration, i.e. of zero order, and for this reason occurs similarly in normals and in hemizygotes. Estimates of the extent of this zero order metabolism can be obtained from turnover data with normal and dysfunctional C1(-)-inhibitor and the results are consistent with the observed levels. In the form of the disease associated with the dysfunction protein the dysfunctional protein makes up more than 85% of the total protein found for the same reason. The extent of the enzyme inhibitor complex dependent catabolism (RO) can be determined in vivo by simultaneous turnovers of dysfunctional and normal inhibitor and gives a measure of the extent of activation of this group of enzymes. The value of this technique in clinical practice is described elsewhere.
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