Changes of the main isoform of human apolipoprotein A-I following incubation of plasma.

This study was aimed to ascertain whether the more acidic isoforms of plasma apo A-I (A-I-1 and A-I-2) could originate in vitro from the main plasma isoform (A-I0). Apo A-I isoforms were separated by two-dimensional gel electrophoresis before and after a prolonged incubation of serum or EDTA-plasma at 37 degrees C. Incubated plasma there was a marked decrease of apo A-I0 and a concomitant linear increase of apo A-I-1 + and A-I-2. The relative content of the latter raised from 22 +/- 7% before the incubation to 60 +/- 7% after 48 h of incubation. This conversion of A-I0 was not inhibited by either pre-heating of plasma at 60 degrees C for 1 h or the addition of protease inhibitors, EDTA and p-chloromercuriphenylsulfonic acid. The conversion of apo A-I0 was not observed in isolated HDL, incubated either in the absence or in the presence of d less than 1.063 g/ml lipoproteins and lipoprotein deficient plasma, nor in plasma which had been dialyzed before being incubated at 37 degrees C. This suggests that plasma contains a low molecular weight factor capable of promoting the conversion of A-I0. The increase of the relative content of apo A-I-1 and apo A-I-2 in incubated plasma was not due to glucosylation or carbamylation of A-I0 as no radioactive glucose and urea were found to be bound to A-I. Since the conversion of apo A-I0 was prevented by the addition of an antioxidant (butylated hydroxytoluene, BHT) to the incubated plasma it is conceivable that some product of lipid peroxidation renders apo A-I0 more electronegative by reacting with some free amino groups of this peptide.