Human serum albumin (HSA) reduced the
phospholipid hydroperoxide, 1-palmitoyl-2-(13-hydroperoxy-cis-9, trans-11-octadecadienoyl)-l-3-phosphatidylcholine (
PLPC-OOH) to the corresponding hydroxy-derivative with a high apparent affinity (Km=9. 23+/-0.95 microM). Removal of bound
lipid during purification increased this activity. At physiological concentration, HSA reduced the
phospholipid hydroperoxide in the absence of a cofactor. However, in the presence of a cofactor (
reductant), the rate of the reaction was increased. All of the major aminothiols in plasma could act as
reductants, the best being the most abundant,
cysteine (Km=600+/-80 microM). For every nanomole of
PLPC-OOH reduced by HSA, 1.26 nmol of
cystine was formed, indicating a reaction stoichiometry of 1 mol
PLPC-OOH to 2 mol
cysteine. We used chemical modification to determine which
amino acid residues on HSA were responsible for the activity. Oxidation of
thiol group(s) by
N-ethylmaleimide led to a reduction in the rate of activity, whereas reduction of
thiols by either
dithiothreitol or the
angiotensin-converting enzyme inhibitor,
captopril, increased the activity. Both
N-ethylmaleimide-modified HSA and
dithiothreitol-treated HSA exhibited increased apparent affinities for
PLPC-OOH. For a range of preparations of
albumin with different modifications, the activity on
PLPC-OOH was dependent on the amount of free
thiol groups on the
albumin (correlation coefficient=0.91). Patients with lowered
albumin concentrations after
septic shock showed lowered total plasma
thiol concentrations and decreased
phospholipid hydroperoxide cysteine peroxidase (
PHCPx) activities. These results therefore show for the first time that HSA exhibits
PHCPx activity, and that the majority of the activity depends on the presence of reduced
thiol group(s) on the
albumin.