Table 4.

Table summarizing substrate specificity, Kms and tissue distribution of ENTs.

ENTs	Km values (Adenosine)	Substrate specificity	Tissue distribution	References
ENT1	40 μM	Adenosine, purine and pyrimidines nucleosides	Ubiquitous. Predominant in adrenal gland, kidneys, stomach, small intestine. Also present in colon and ovaries.	(Griffith and Jarvis, 1996; Kong et al. 2004; Inoue, 2017)
ENT2	100 μM	Adenosine, purine and pyrimidines nucleosides, nucleobases	Ubiquitous. Highest in skeletal muscles. Also abundant in brain, neural tissues, heart, placenta, GI tract	(Griffith and Jarvis, 1996; Kong et al. 2004)
ENT3	2 mM	Adenosine, purine and pyrimidines nucleosides	Intracellular membranes, cerebral cortex, ovary, adrenal gland, and testis.	(Hyde et al. 2001; Baldwin et al. 2005)
ENT4	0.78 mM	Adenosine, serotonin	Abundant in heart, especially vascular endothelial cells.	(Baldwin et al. 2004; Barnes et al. 2006)

Abbreviations: ENT, Equilibrative nucleotide transporter; GI, Gastrointestinal.