Information accumulated over the past decades on the physiological functions and metabolic pathways of biosynthesis and degradation of D-amino acids has led to a renewed interest in their study. These isomers are known to form both in nature and during the chemical synthesis of L-amino acids for feeding and pharmacological purposes, as well as in the industrial processing of some raw materials. This article discusses the positive and negative effects of D-amino acids on the human body, animals and the environment. In addition, the scientific data concerning the mechanisms of cytotoxic action of D-amino acids and their industrial and biomedical potential are summarized. ARTICLE HISTORY

Site-directed mutagenesis of Rhodospirillum rubrum L-asparaginase (RrA) was performed in order to identify sites of the protein molecule important for its therapeutic and physico-chemical properties. Ten multipoint mutant genes were obtained, and five recombinant RrA variants were expressed in E. coli BL21(DE3) cells and isolated as functionally active highly purified proteins. Protein purification was performed using Q-Sepharose and DEAE-Toyopearl chromatography. Overall yield of the active enzymes was 70-80 %, their specific activity at pH 7.4 and 37 °C varied of 140-210 U/mg. L-Glutaminase activity did not exceed 0.01 % of L-asparaginase activity. All RrA mutants showed maximum enzyme activity at pH 9.3-9.5 and 53-58 °C. Km and Vmax values for L-asparagine were evaluated for all mutants. Mutations G86P, D88H, M90K (RrAH), G121L, D123A (RrАI) caused the loss of enzyme activity and confirmed the importance of these sites in the implementation of catalytic functions. Removal of four residues from C-terminal area of the enzyme (RrAK) resulted in the enzyme instability. Mutations D60K, F61L(RrАD), and R118H, G120R(RrАJ) led to the improvement of kinetic parameters and enzyme stabilization. Substitutions E149R, V150P (RrАB) improved antineoplastic and cytotoxic activity of the RrA. A64V, E67K substitutions, especially in combination with E149R, V150P (RrАE), considerably destabilized recombinant enzyme.

Rhodospirillum rubrum L‐asparaginase mutant E149R, V150P, F151T (RrA) down‐regulates telomerase activity due to its ability to inhibit the expression of telomerase catalytic subunit hTERT. The aim of this study was to define the effect of short‐term and long‐term RrA exposure on proliferation of cancer Jurkat cell line and normal human CD4+ T lymphocytes. RrA could inhibit telomerase activity in dose‐ and time‐dependent manner in both Jurkat and normal CD4+ T cells. Continuous RrA exposure of these cells resulted in shortening of telomeres followed by cell cycle inhibition, replicative senescence, and development of apoptosis. Complete death of Jurkat cells was observed at the day 25 of RrA exposure while normal CD4+ T cells died at the day 50 due to the initial longer length of telomeres. Removal of RrA from senescent cells led to a reactivation of hTERT expression, restoration telomerase activity, re‐elongation of telomeres after 48 h of cultivation, and survival of cells. These findings demonstrate that proliferation of cancer and normal telomerase‐positive cells can be limited by continuous telomerase inhibition with RrA. Longer telomeres of normal CD4+ T lymphocytes make such cells more sustainable to RrA exposure that could give them an advantage during anti‐telomerase therapy. These results should facilitate further investigations of RrA as a potent anti‐telomerase therapeutic protein.

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Main forms of cellular signal transmission are known to be autocrine and paracrine signaling. Several cells secrete messengers called autocrine or paracrine agents that can bind the corresponding receptors on the surface of the cells themselves or their microenvironment. Follistatin and follistatin-like proteins can be called one of the most important bifunctional messengers capable of displaying both autocrine and paracrine activity. Whilst they are not as diverse as protein hormones or protein kinases, there are only five types of proteins. However, unlike protein kinases, there are no minor proteins among them; each follistatin-like protein performs an important physiological function. These proteins are involved in a variety of signaling pathways and biological processes, having the ability to bind to receptors such as DIP2A, TLR4, BMP and some others. The activation or experimentally induced knockout of the protein-coding genes often leads to fatal consequences for individual cells and the whole body as follistatin-like proteins indirectly regulate the cell cycle, tissue differentiation, metabolic pathways, and participate in the transmission chains of the pro-inflammatory intracellular signal. Abnormal course of these processes can cause the development of oncology or apoptosis, programmed cell death. There is still no comprehensive understanding of the spectrum of mechanisms of action of follistatin-like proteins, so the systematization and study of their cellular functions and regulation is an important direction of modern molecular and cell biology. Therefore, this review focuses on follistatin-related proteins that affect multiple targets and have direct or indirect effects on cellular signaling pathways, as well as to characterize the directions of their practical application in the field of biomedicine.