Carbonyl proteins as a clinical marker in Alzheimer's disease and its relation to tryptophan degradation and immune activation.

BACKGROUND

The question arises whether oxidative stress is connected with systemic immune activation in Alzheimer's disease (AD) and mild cognitive impairment (MCI). During immune response interferon-gamma stimulates the kynurenine (Kyn) pathway, a major route of L-tryptophan (Trp) degradation.

METHODS

Plasma Kyn, Trp and the Kyn to Trp ratio (Kyn/Trp), carbonyl proteins (CP) as oxidative stress parameter and homocysteine, neopterin, folate and vitamin B12 were measured from patients with AD and MCI (n = 16: 6 females and 4 males with AD, 3 females and 3 males with MCI; 63.3 +/- 13.7 years), and an age matched healthy control group (n = 15: 11 females and 4 males; 62.8 +/- 3.6 years). We correlated the oxidative stress parameter CP with the degradation of Trp creating a new quotient CP/Trp and calculated the sensitivity, specificity, and cut-off values for CP, Trp, CP/Trp, and Kyn/Trp using discriminate analysis.

RESULTS

CP was significantly higher in AD/MCI (930 +/- 265 pmol/mg; p < 0.001) compared to controls (300 +/- 120 pmol/mg), Trp was significantly lower in AD/MCI (48.9 +/- 9.0 micromol/L; p < 0.001) than controls (65.2 +/- 10.7 micromol/L). While Kyn showed no significant difference between AD/MCI (1.72 +/- 0.56 micromol/L) and controls (1.53 +/- 0.29 micromol/L), Kyn/Trp was significantly higher in AD/MCI (35.2 +/- 8.8 micromol/mmol; p < 0.001) than in controls (23.7 +/- 4.2 micromol/mmol). CP/Trp ratio was more than 4 fold higher in the AD/MCI group (19.8 +/- 7.76 [(pmol/mg)/(micromol/L)]; p < 0.001) compared to controls (4.79 +/- 2.26 [(pmol/mg)/(micromol/L)]). Homocysteine, folate, vitamin B12, and neopterin showed no significant difference. Discriminant analysis provided CP alone as the best clinical marker with highest sensitivity and highest specificity for AD/MCI followed by the ratio of CP/Trp. ROC curve analysis provided the best result for CP/Trp.

CONCLUSIONS

These preliminary results support the hypothesis that oxidative damage to proteins is directly connected with Trp degradation and Kyn pathway in the systemic immune activation.