Table 2.

Different classes of selenoproteins along with their function in different cells.

S. No	Selenoprotein	Function	Tissue localization	Ref.
1.	Glutathione peroxidase-1 (GPx-1)	Antioxidant, inhibits mutations in virus by decreasing the ability of virulence in retrovirus	Present in liver, kidneys, and lungs	[63]
2.	Glutathione peroxidase-2 (GPx-2)	Antioxidant, prevents oxidative stress, maintains integrity of intestinal mucosa, anti-apoptotic activity in colon	Present mainly in liver and gastrointestinal tract	[64]
3.	Glutathione peroxidase-3 (GPx-3)	Decreased lipid hydro peroxides, exhibited antioxidant activity, protects thyroid gland from hydrogen peroxide	Present in extracellular fluid, breast, plasma, liver, placenta heart, gastro-intestinal tract, male reproductive system and kidneys	[65,66]
4.	Glutathione peroxidase-4 (GPx-4)	Antioxidant activity prevents brain from peroxidative damage, converts cholesterol and cholesterol esters into lower toxic analogs, crucial in sperm viability and motility, regulation of follicular helper T cells homeostasis	Present in testes	[[67], [68], [69]]
5.	Glutathione peroxidase-5 (GPx-5)	Unknown	Olfactory lining and embryo	[70]
6.	Glutathione peroxidase-6 (GPx-6)	Unknown	Present in humans only	[71]
7.	Glutathione peroxidase-7 (GPx-7)	Reverses linkage among GPx-7 and cancer cells multiplication	Lumen	[71,72]
8.	Glutathione peroxidase-8 (GPx-8)	Role in protein folding, antioxidant	Liver, kidney	[72]
9.	Iodothyronine deiodinase-1 (DIO1)	Generation of active T3 hormones in thyroid as well as in peripheral tissues. Conversion of inactive thyroxin into active tri-iodothyronine	Mainly in thyroid, kidney, brown fat and liver	[73]
10.	Iodothyronine deiodinase-2 (DIO2)	T3 generation in peripheral tissues, induction of thyroid hormones	Present in central nervous system, heart, brown adipose tissue, pituitary, and skeletal muscle	[74]
11.	Iodothyronine deiodinase-3 (DIO3)	Inhibits presentation of foetus to T3cells, inactivity of thyroid hormones	Found in cerebral cortex, uterus, placenta, skin, fetal, and central nervous system (CNS)	[75]
12.	Thioredoxin reductase 1 (TXNR1)	Antioxidant function, decrease thioredoxin, controls apoptosis, transcription factors, and cell proliferation	Liver, testes	[63]
13.	Thioredoxin reductase 2 (TXNR2)	Cell growth factor in DNA synthesis and inhibition of apoptosis	Liver, kidneys	[63]
14.	Thioredoxin reductase 3 (TXNR3)	Unknown	Present in testes
15.	Selenoprotein P (SELENOP)	Transport Se, maintain homeostasis, antioxidant activity, possesses ten selenocysteine moiety	Predominantly present in plasma, liver, testes, and brain	[76,77]
16.	Selenoprotein S (SELENOS)	Role in inflammation, clears out mis-folded ER proteins, stimulates apoptosis, ER stress	Liver, kidney	[78]
17.	Selenoprotein N (SELENON)	Redox signaling, calcium homeostasis and muscle development	Liver, kidney	[79]
18.	Selenoprotein W (SELENOW)	Antioxidant activity in lungs, calcium attaching ability, regulates differentiation of osteoclast and inhibition of osteoporosis, regulation of T cells activity	Skeletal muscle, prostate, heart, colon, long bone, brain, liver and brain	[[80], [81], [82]]
19.	Selenoprotein K (SELENOK)	Antioxidant activity, myogenesis, regulation of ER stress	Immune cells and spleen, skeletal muscle	[[83], [84], [85]]
20.	Selenoprotein H (SELENOH)	Gene regulation of glutathione	Brain, and muscle cells	[86]
21.	Selenoprotein R (SELENOR)	Antioxidant, protein repair, methionine metabolism	Kidney and liver cells	[87]
22.	Selenoprotein M (SELENOM)	Antioxidant activity	Neuronal cells	[80]
23.	15Kd selenoprotein (SEP15)	Role in folding of glycoprotein	Kidney and liver cells	[88]
24.	Selenophosphate synthase 2 (SPS-2)	Selenocysteine biosynthesis from selenophosphates	Kidney, liver	[89]