title: Enzymes involved in L-carnitine biosynthesis are expressed by small intestinal enterocytes in mice: implications for gut health.
abstract: BACKGROUND
  Carnitine is essential for mitochondrial β-oxidation of long-chain fatty acids. Deficiency of carnitine leads to severe gut atrophy, ulceration and inflammation in animal models of carnitine deficiency. Genetic studies in large populations have linked mutations in the carnitine transporters OCTN1 and OCTN2 with Crohn's disease (CD), while other studies at the same time have failed to show a similar association and report normal serum carnitine levels in CD patients.
  
  
  METHODS
  In this report, we have studied the expression of carnitine-synthesizing enzymes in intestinal epithelial cells to determine the capability of these cells to synthesize carnitine de novo. We studied expression of five enzymes involved in carnitine biosynthesis, namely 6-N-trimethyllysine dioxygenase (TMLD), 4-trimethylaminobutyraldehyde dehydrogenase (TMABADH), serine hydroxymethyltransferase 1 and 2 (SHMT1 and 2) and γ-butyrobetaine hydroxylase (BBH) by real-time PCR in mice (C3H strain). We also measured activity of γ-BBH in the intestine using an ex vivo assay and localized its expression by in situ hybridization.
  
  
  RESULTS
  Our investigations show that mouse intestinal epithelium expresses all five enzymes required for de novo carnitine biosynthesis; the expression is localized mainly in villous surface epithelial cells throughout the intestine. The final rate-limiting enzyme γ-BBH is highly active in the small intestine; its activity was 9.7 ± 3.5 pmol/mg/min, compared to 22.7 ± 7.3 pmol/mg/min in the liver.
  
  
  CONCLUSIONS
  We conclude that mouse gut epithelium is able to synthesize carnitine de novo. This capacity to synthesize carnitine in the intestine may play an important role in gut health and can help explain lack of clinical carnitine deficiency signs in subjects with mutations with OCTN transporters.
authors.affiliation: Department of Pediatrics, Medical College of Georgia, Georgia Health Sciences University, Augusta, GA 30912, United States. shekhawatp@ecu.edu
authors.forename: Prem S
authors.initials: PS
authors.lastname: Shekhawat
authors.forename: Srinivas
authors.initials: S
authors.lastname: Sonne
authors.forename: A Lee
authors.initials: AL
authors.lastname: Carter
authors.forename: Dietrich
authors.initials: D
authors.lastname: Matern
authors.forename: Vadivel
authors.initials: V
authors.lastname: Ganapathy
mesh_terms: D000445:Aldehyde Oxidoreductases / Q000235:genetics / Q000378:metabolism; D000818:Animals; D002331:Carnitine / Q000032:analysis / Q000096:biosynthesis*; D020895:Enterocytes / Q000201:enzymology*; D015870:Gene Expression; D012696:Glycine Hydroxymethyltransferase / Q000235:genetics / Q000378:metabolism; D017403:In Situ Hybridization; D007413:Intestinal Mucosa / Q000737:chemistry / Q000201:enzymology*; D007421:Intestine, Small / Q000737:chemistry / Q000201:enzymology*; D051379:Mice; D008809:Mice, Inbred C3H; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D050563:gamma-Butyrobetaine Dioxygenase / Q000235:genetics / Q000378:metabolism
chemical_list: D006899:Mixed Function Oxygenases; D050563:gamma-Butyrobetaine Dioxygenase; C436318:epsilon-N-trimethyllysine hydroxylase; C023095:4-N-trimethylaminobutyraldehyde dehydrogenase; D000445:Aldehyde Oxidoreductases; D012696:Glycine Hydroxymethyltransferase; C559685:Shmt1 protein, mouse; D002331:Carnitine