New method for the estimation of platelet ascorbic acid

Plasma and platelet ascorbate pools and lipid peroxidation in insulin-dependent diabetes mellitus

BACKGROUND

As diabetes mellitus represents a situation in which production of peroxides is increased, the aim of this study was to investigate the relationship between plasma and platelet levels of ascorbic acid (AA)/dehydroascorbic acid (DHA) and those of malonyldialdehyde (MDA), an indirect marker of lipoperoxides, both assayed using high-performance liquid chromatography (HPLC), in 59 patients with insulin-dependent diabetes mellitus (IDDM) compared with 51 healthy control subjects matched for sex, age, smoking habits, as well as for dietary intake of energy, alcohol and vitamin C.

RESULTS

Mean plasma and platelet MDA were significantly higher in the patients affected with IDDM than in control subjects. Moreover, the diabetic group was characterized by a huge decrease in plasma AA [8.45 +/- 5.5 mumol L-1 (SD) vs. 33.4 +/- 7.6 mumol L-1, P = 0.0001], mirrored by a significant increase in plasma DHA (11.9 +/- 3.9 mumol L-1 vs. 3.9 +/- 2.5 mumol L-1, P = 0.0001). No detectable DHA was observed in the platelets from both diabetic and control subjects, whereas AA was significantly increased in platelets from diabetic patients compared with control subjects (42.6 +/- 7.4 vs. 34.8 +/- 5.1 nmol 10(-9) platelets, P = 0.0001). Platelet AA in the diabetic group was significantly inversely correlated with glycated haemoglobin (r = -0.34; P = 0.04) and directly with plasma AA (r = 0.39; P = 0.02), the sum of plasma AA + DHA (r = 0.44; P = 0.009) and with platelet MDA (r = 0.38; P = 0.02).

CONCLUSION

(a) The ratio plasma AA/DHA is significantly lowered in IDDM in association with an increase in MDA levels; (b) only AA is detected in platelets, being augmented in the diabetic group; (c) plasma ascorbate depletion does not reflect platelet levels of AA; and, finally, (d) metabolic control, as well as intracellular lipoperoxides, modulates platelet AA in IDDM.

A rapid manual method for routine assay of ascorbic acid in serum and plasma

A method is described for the assay of ascorbic acid in either serum or heparinized plasma. 1. The assay is based on the reduction of ferric chloride by ascorbic acid with the resulting ferrous ion quantitated by the addition of 2,4,6-tripyridyl-s-triazine to form a purple colour with a maximum absorbance at 595 nm. 2. Uric acid interference has been eliminated by the use of a high molarity acetate buffer and by optimising the amount of TPTZ and ferric chloride used. 3. Protein was found to cause rapid fading of the final colour; it was therefore necessary to remove the protein, by addition of 10% trichloroacetic acid, from the specimen prior to the final assay. This had the added advantage of assisting to stabilize the ascorbic acid prior to final assay. 4. All reagents used are easily obtained and no special equipment is required.

Platelet ascorbic acid levels in normal subjects and in disease

The platelet ascorbic acid concentration was measured in 26 normal subjects and found to be 20 times as high as in plasma. This is in agreement with previous reports in the literature. The platelets of patients with uraemia, leukaemia, and megaloblastic anaemia had a lower than normal platelet ascorbic acid content. In uraemia and megaloblastic anaemia the plasma ascorbic acid concentration was normal suggesting that a platelet defect may be responsible for the low platelet ascorbic acid content. In leukaemia the low platelet ascorbic acid content is probably secondary to a low plasma level.