Changes in the level of some tryptophan metabolites in the blood of patients with type 2 diabetes mellitus complicated by diabetic retinopathy

Abstract

Aim: To study the content of tryptophan and the intermediate metabolites of kynurenine and serotonin pathways of its metabolism in the blood plasma of patients with type 2 diabetes mellitus (DM) and non-proliferative diabetic retinopathy as candidates for markers of the early stage of the pathological process.Material and Methods. Three groups of people were analyzed: the first group of 10 patients with type 2 diabetes mellitus and without diabetic retinopathy; the second group of 10 people with type 2 diabetes mellitus and non-proliferative stage of diabetic retinopathy; and the control group of 10 healthy people. The features of the first group were a decrease of macula’s photosensitivity in the fundus and a slight increase in the central thickness of the retina. Patients with diabetic retinopathy tended to have a moderate number of microaneurysms and microhemorrhages, moderately presented intraretinal microvascular anomalies in one quadrant, vein dilatation, clearness of the central retinal vein and its branches. In the macular zone there was an edema with hard exudates in the center and lateral to the fovea centralis. All participants of the study gave blood on an empty stomach in the morning, and after that in we measured the content of tryptophan (TRP), kynurenins ((kynurenine (KYN), 3-hydroxykynurenine (3-HKYN), kynurenic acid (KYNA)) and the level of L-5-hydroxytyrptophan (5HTrp) in blood plasma by HPLC method with fluorimetric and spectrophotometric detection.Results. The groups of people with type 2 diabetes mellitus showed the increasing of TRP level in blood relatively to healthy individuals: by 15.1% (p = 0.032) and 17.9% (p = 0.030) in the first and second groups, respectively. As for the patients of the first group, the content of their KYN was increased by 57.7% (p = 0.012) and KYNA by 33.6% (p = 0.012) relatively to the control and the concentration of 3-НKYN decreased by 18.1% (p = 0.020) relatively to healthy people. As for the patients in the second group, the changes in their level of kynurenines had the same direction, but were more visible. Thus, the concentration of KYN exceeded the same parameters of healthy individuals by 84.5% (p = 0.001) and the parameters of the first group by 18.0% (p = 0.049); the KYNA level increased by 56.6% (p = 0.001) relatively to the control and by 17.3% (p = 0.049) from that of the first group. There was a decrease in the content of 3-HKYN amounted to 18.6% of the control (p = 0.038) and an increase in the concentration of 5HTrp – 193,9% (p < 0.001).