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Both members of the novel OCT (OCTN) family are highly expressed in the kidney and are more ubiquitously expressed at low levels across a variety of tissues [1] (Table 2.1). OCTN1 is expressed most highly in whole blood, particularly within erythrocytes. It has broad tissue distribution at low levels, notably in the brain, small intestine, lung, and reproductive organs. In mice, Octn1 has been localized to the apical membrane of the renal proximal tubule. Consistent with the role of a transporter for the antioxidant ergothioneine, OCTN1 is expressed in tissues exposed to high oxidative stress, notably the kidney, liver, bone marrow/erythrocytes, eye lens, and seminal fluid [2]. Unlike the majority of related zwitterion transporters, OCTN1 appears to play a minimal role in the transport of carnitine. In humans, OCTN2 is expressed ubiquitously at low levels in most tissues. Highest expression is observed in skeletal muscle, brain, kidney, intestine, cardiac tissue, and reproductive organs. Many of these tissues have high energy demands and rely heavily on fatty acid β‐oxidation for ATP production. OCTN2 expression in these tissues ensures carnitine stores are available to conjugate to intracellular long‐chain fatty acids for translocation into the mitochondrial matrix where β‐oxidation occurs. In the kidney, OCTN2 is localized to the apical membrane of the renal proximal tubule where it functions largely in the reabsorption of renally excreted carnitine from urine to maintain systemic levels.

In humans, SLC22A15 is ubiquitously expressed throughout all tissues in the body, although highest expression has been observed in the brain and bone marrow. Other species exhibit similar expression patterns, including primates (baboon, rhesus macaque, old world monkey), cattle, pigs, sheep, mice, rats, chickens, some lizards, and zebrafish [5]. Human SLC22A15 is mainly involved in the transport of ergothioneine, like OCTN1, and supports transport of other zwitterions and cations. CT2 is another high‐affinity carnitine transporter, but in contrast to OCTN2 and SLC22A15, expression is limited to the Sertoli cells of the testis, the kidneys, and bone marrow [1]. These tissues have high energy requirements, especially the testis, where carnitine plays a crucial role in spermatogenesis. Similar CT2 expression patterns are also found in primates, mice, cattle, and chicken.