| Lipoprotein(a) Reduction
by Ascorbate (1992)
Rath M. Journal of Orthomolecular Medicine
(1992), 7: 81-82.
Published in:
Journal of Orthomolecular Medicine 7: 81-82
High plasma levels of lipoprotein(a) constitute a
strong risk factor for cardiovascular disease. Lipoprotein(a) is
primarily found in the plasma of man and other species that are unable
to synthesize ascorbate endogenously. Here it is shown that ascorbate,
a strong physiological reducing agent, lowers elevated lipoprotein(a)
plasma levels in man.
Introduction
Lipoprotein(a) [Lp(a)] is associated with an
increased risk of atherogenesis and thrombogenesis. Recently it
was proposed that Lp(a) is a surrogate for ascorbate. (1) This proposal
suggested a role of ascorbate in the regulation of Lp(a) synthesis:
namely, that increased intake of ascorbate, a strong natural reducing
agent, would lower Lp(a) plasma levels. N-Acetylcysteine (NAC) was
then also proposed to lower Lp(a) plasma levels and was reported
to do this to a variable degree. (2,3) The effect of ascorbate in
lowering Lp(a) plasma levels was studied in a clinical pilot study
with the results reported here. Patients,
Materials and Methods
Eleven outpatients with coronary heart disease and elevated Lp(a)
levels consented to participate in this study. The patients received
9 grams of ascorbic acid (Bronson Pharmaceuticals, La Canada,
California) per day for a period of 14 weeks. Plasma Lp(a) levels
were determined at the beginning and at the end of the study.
Lp(a) plasma levels were determined by a sandwich ELISA method
with monoclonal capture antibodies against apo(a) and monoclonal
peroxidase-labeled antibodies against the apoB-100 portion of
the Lp(a) molecule. (4) The antibodies were a gift from Dr. J.
C. Fruchart, Lille, France.
Results
In this study ascorbate was found to lower Lp(a) plasma levels
on average by 27% with a median value also of 27% (Table 1). Two
of the 11 patients showed no decrease of Lp(a) during this time
period. Lp(a) in the same plasma samples was also measured with
immunological assays using monoclonal antibodies against the apo(a)
portion of the Lp(a) molecule for both, capturing and revealing
(radioimmunoassay[RIA], Pharmacia Diagnostics; anti-apo(a) sandwich
ELISA). Changes in Lp(a) plasma concentrations were measured for
RIA mean +2%, median -7.5 % and for ELISA mean -4%, median -12%.
The mean values for vitamin C plasma levels were 48.6 uM at the
beginning and 94.4 uM at the end of the study.
Discussion
Two factors may account for the differences between the assay
including an antibody against apoB and the assays using exclusively
anti-apo(a) antibodies. One factor could be the variation in epitopes
of the apo(a) molecule as a result of the variation of the molecular
size determined by the genetic isoforms. This factor was largely
excluded in this study by determining the apo(a) isoforms by means
of SDS PAGE and subsequent immunoblotting with anti apo(a) antibodies.
The second possible factor accounting for these
differences is the effect of reducing agents on the intramolecular
disulfide bonds of the apo(a) molecule. This factor is discussed
here in more detail. Apo(a) has been proposed to function as a
proteinthiol (1) and the disulfide bonds of the repetitive plasminogen
kringle IV structure are known to have different dissociation
constants. Elevated plasma concentrations of reducing agents such
as ascorbate or NAC could alter the epitope constellation of the
apo(a) molecule in vivo by reducing some of the many disulfide
bonds to sulfhydryl groups. Under this condition, assays using
only anti-apo(a) antibodies could give falsely positive results,
dependent on the specific epitopes they recognize in the repetitive
kringle structures of the apo(a) molecule.
In contrast, an assay measuring the apoB portion
of the Lp(a) molecule should provide more reliable results since
apoB contains less disulfide bonds and in addition has a constant
molecular size. This conclusion could also explain the fact that
the only two studies reporting a lowering of Lp(a) plasma levels
with reducing agents included assays using anti-apoB antibodies
for detection (2, and this paper). In contrast, assays exclusively
based on antibodies against apo(a) gave variable results in the
presence of reducing agents. (2,3)
From in vitro studies with NAC it was
recently concluded that supraphysiological concentrations of reducing
agents above 1 mM decrease the immunoreactivity for Lp(a). (5)
The extrapolation of these results to the in vivo situation must,
of course, be handled with care. The highest molar concentration
of ascorbate measured in the study reported here was 154 uM, a
level that does not decrease the immunoreactivity of apo(a) or
Lp(a). The effect of physiological levels of ascorbate on the
reduction of disulfide bonds of the apo(a) molecule as well as
the possible immunological implications need further investigation.
The results of the clinical study reported here,
namely that dietary ascorbate supplementation reduces Lp(a) plasma
levels, was supported by in vitro studies in our laboratory with
human liver cells (HepG2, data not shown). In metabolic studies
using S 35 methionine increasing concentrations of ascorbate in
the cell culture medium decreased the amount of Lp(a) secreted
by these cells. Ascorbate concentrations up to 2.25 mM did not
reveal any dissociation of apo(a) from apoB. It is, therefore,
concluded that the effect of ascorbate on Lp(a) plasma levels
is the result of a decreased rate of synthesis of Lp(a) particles
in the liver.
In conclusion, ascorbate is a physiological reducing agent involved
in the metabolic regulation of Lp(a) synthesis. Dietary supplementation
of ascorbate, as an adjunct to conventional therapy, should contribute
to reducing elevated Lp(a) plasma levels and the risk of cardiovascular
disease. Prolonged supplementation of ascorbate may be required
to achieve these effects..
References
1. Rath M, Pauling L. Hypothesis: Lipoprotein(a) is a surrogate
for ascorbate. Proceedings of the National Academy of Sciences
USA 1990; 87: 6204-6207
2. Gavish D, Breslow J. Lipoprotein(a) reduction
by N-acetylcysteine. Lancet 1991; 337:203-204
3. Stalenhoef AFH, Kroon A, Demacker PNM. N-acetylcysteine
and lipoprotein. Lancet 199 1; 337: 491
4. Vu Dac N, Mezdour H, Parra HJ, Luc G, Luyeye
I, Fruchart JC. A selective bi-site immunoenzymatic procedure
for human Lp(a) lipoprotein quantification using monclonal antibodies
against apo(a) and apoB. Journal of Lipid Research 1989;
30: 1437-1443
5. Scanu AM, Pfaffinger D, Fless GM, Makino K,
Eisenbart J, Hinman J. Attenuation of immunologic reactivity of
lipoprotein(a) by thiols and cysteine-containing compounds. Arteriosclerosis
and Thrombosis 1992; 12: 424-429
|
