SULFHYDRYL COMPOUNDS INFLUENCE IMMUNOREACTIVITY, STRUCTURE AND FUNCTIONAL-ASPECTS OF LIPOPROTEIN(A)

Human plasma Lp(a) is susceptible to various sulfhydryl compounds. In this study we present evidence indicating that after treatment of Lp(a ) with sulfhydryl compounds, immunoreactivity is changed, structural c hanges occur and functional characteristics regarding the numerous kri ngle structures in apo(a) disappear. Purified Lp(a) was subjected to v ariable concentrations (0.01 - 10 mM) of various sulfhydryl compounds: DTT, 2-mercapto-ethanol (BME), N-acetylcysteine (NAC) and homocystein e (HCys). Free SH groups were blocked by iodoacetamide. Reduced and al kylated Lp(a) was tested in two ELISAs, one detecting apo(a) alone and one detecting apo(a)-apoB complexes. In both ELISAs polyclonal antibo dies were used. For comparison a commercial apo(a) IRMA utilizing two monoclonal antibodies was used. The results indicate that a similar de crease in response of both ELISAs is observed, whereas the IRMA respon se is less affected. Western blotting of ''DTT treated'' Lp(a) after S DS-PAGE under nonreducing conditions showed that separate apo(a) and a poB-100 bands became detectable at 1 mM DTT. Native PAGE (2.5 - 16%) i ndicated structural changes of Lp(a) beginning to occur at 0.03 mM DTT . E-aminocaproic acid-inhibitable binding of ''DTT-treated'' Lp(a) to Desafib-X decreased with increasing DTT concentrations in concert with a loss of the capacity of Lp(a) to inhibit plasminogen activation upo n treatment with DTT. The observed immunological and functional change s of Lp(a) indicate that apo(a) kringle function is severely affected by sulfhydryl compounds.