Abstract:Cerebrospinal fluid (CSF) from 20 male patients with nonneurologic disease (age 64.5 ± 2.8 SEM) was analyzed for the presence of the serpin α1‐antichymotrypsin (α1‐ACT). A chymotrypsin‐specific chromogenic substrate (succinyl‐Ala‐Ala‐Pro‐Phe‐p‐nitroanilide) was used to examine the CSF samples. All CSF samples showed inhibitory activity ranging from 45 to 80% inhibition. Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis analysis of the samples revealed the presence of a 68‐kDa protein migrating identical to authentic human plasma α1‐ACT. Complex formation was performed with iodinated bovine chymotrypsin for several representative CSF samples having the highest chymotrypsin inhibitory activity. Comparison was made with complex formation performed with commercially available authentic human plasma α1‐ACT. These studies showed the formation of complexes at 37°C, regardless of whether the sample was subsequently boiled or not. In the case of CSF, two complex bands, mass smaller than with plasma α1‐ACT, were formed at the lower temperature whereas a single higher Mrband was formed when the samples were boiled. To determine whether cleavage of the serpin occurred, these studies were repeated using human neutrophil cathepsin G as target protease. A complex of approximately 90 kDa was formed with human α1‐ACT under these same conditions. α1‐ACT has been detected in senile amyloid plaques in brains of Alzheimer's disease patients, the only plasma serine protease inhibitor localized to these structures. Another serpin, protease nexin I, is also found in these plaques, but this inhibitor does not circulate in plasma. Recently, the β‐amyloid precursor protein itself has been identified as another serine protease inhibitor, protease nexin II, known to form complexes with bovine chymotrypsin as well as with the epidermal growth factor‐binding protein. Protease nexin II does circulate in plasma; however, complexes of this inhibitor with chymotrypsin are twice as large as the complexes this protease forms with plasma α1‐ACT or with the CSF samples. Using antisera to the β‐amyloid precursor protein, other workers have detected both 125‐kDa and 105‐kDa proteins in human CSF, the 125 kDa reportedly detected by antisera against the Kunitz serine protease inhibitor‐containing domain. The studies with α1‐ACT in CSF should now allow us to evaluate this serpin and its function in CSF of pa