Albumin molecule, unlike molecules of many other plasma proteins, is not covered with carbohydrate shell. It plays a crucial role in maintaining of colloid osmotic pressure of the blood, and is able to bind and transport various endogenous and exogenous molecules. The enzymatic activity of albumin, the existence and the role of which most researchers are still skeptical to accept, is of the main interest to us. In this review, a history of the issue is traced, with particular attention to the esterase activity of albumin. The kinetic and thermodynamic characteristics of the interaction of albumin with some substrates are adduced, and possibility of albumin being attributed to certain groups of Enzyme Nomenclature is considered.

Ранее установлено, что внеклеточная глутатионпероксидаза (GPx3) из плазмы человека способна утилизировать цистеин (Cys-SH) в качестве тиолового субстрата вместо глутатиона (GSH). В настоящей работе подтверждена гипотеза о том, что плазма крови крыс обладает тиолпероксидазной активностью не только по отношению к GSH (глутатионпероксидазная активность), но и по отношению к Cys-SH и гомоцистеину (Hcy-SH). Мольное соотношение между каждым из тиоловых субстратов и пероксидом водорода в катализируемой реакции составляет 2:1, удельная активность возрастает при выделении соответствующей белковой фракции. При фиксированной концентрации тиола (230 мкМ) выявляется насыщение по H 2 O 2 с v max каж , равной 100, 128 и 132 нмоль H 2 O 2 /(с ´мл плазмы) для DL-Cys-SH, L-GSH и DL-Hcy-SH, соответственно. Ранговое распределение активностей со всеми тремя тиоловыми субстратами среди белковых фракций плазмы полностью идентично (вероятность случайного полного совпадения -менее 0,01), что статистически подтверждает тесную связь цистеинпероксидазной, гомоцистеинпероксидазной и глутатионпероксидазной активностей между собой. Наиболее вероятным выводом из этого является способность GPx3 крысы использовать все три тиола в качестве окисляемых субстратов. Возможно, тиолпероксидаза вносит вклад в преобладание цистина (Cys-SS-Cys) над Cys-SH в плазме крови. Можно предположить, что при гипергомоцистеинемии тиолпероксидаза плазмы крови, модулируя редокс-соотношение Hcy, вносит вклад в выраженность токсических эффектов последнего.

A method of measuring of glutathione peroxidase activity using HO was adapted for homogenates of murine brains. If the amount of reduced glutathione was at the constant level of 0.55 mM, the concentration of HO of 0.192 mM was saturating for glutathione peroxidase of murine brain and was selected as an optimal concentration for the estimation of enzyme activity in tris-HCl buffer with addition of NaN and EDTA (pH 8.5) at the incubation temperature of 37°C. The homogenates were dissolved by the reaction mixture by 10.4 times. The duration of incubation did not exceed 60 sec, if 13% homogenate was used. The experiment based on this method showed increased activity of glutathione peroxidase in the brain of mice treated with a derivative of acetaldehyde ammonia during long-term intermittent normobaric hypoxia. These data might reflect activation of glutathione peroxidase.

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Recently it was shown that the presence of rat blood plasma (as well as of erythrocyte hemolysate) in the reaction mixture containing 43 mM Tris-HCl-buffer (pH 8.5), 0.29 mM EDTA, 19.2 mM sodium azide, 1 mM DL-homocysteine (Hcy), and 198 mM hydrogen peroxide (incubation at 37°C) results in a significant acceleration of the decrease in Hcy concentration caused by addition of H O . In this paper, we present data indicating that the observed activity is the homocysteine:H O -oxidoreductase (homocysteine peroxidase) activity. It has been found that the level of H O -dependent Hcy decrease observed in the presence of blood plasma corresponds to homocysteine:H O -oxidoreductase reaction stoichiometry of 2:1 (mole ratio). The activity observed belongs to the protein fraction isolated by saturation with ammonium sulfate to 50%; the specific activity in this protein fraction is significantly higher than that in the whole plasma. The results confirm the hypothesis that the reaction between Hcy and H O at the presence of plasma is catalyzed by the protein component of plasma and this is the homocysteine peroxidase reaction. This activity is not associated with serum albumin, which is known to function as thiol peroxidase, and probably belongs to extracellular glutathione peroxidase (Gpx3).

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