L-histidine

1. 4-aminobutanoate + L-histidine + ATP = L-homocarnosine + ADP + phosphate + H(+)

Catalysed by CARNS1

1. L-histidine(out) + L-arginine(in) + Na(+)(out) = L-histidine(in) + L-arginine(out) + Na(+)(in)

Catalysed by SLC7A6inner side of the membrane, outer side of the membraneOne of 3 routes

1. L-histidine = trans-urocanate + NH4(+)

Catalysed by HAL

1. L-histidine(in) + L-ornithine(out) + H(+)(in) = L-histidine(out) + L-ornithine(in) + H(+)(out)

Catalysed by SLC25A2inner side of the membrane, outer side of the membraneOne of 2 routes

1. beta-alanine + L-histidine + ATP = carnosine + ADP + phosphate + H(+)

Catalysed by CARNS1

1. L-histidine(in) + L-phenylalanine(out) = L-histidine(out) + L-phenylalanine(in)

Catalysed by SLC7A8inner side of the membrane, outer side of the membraneOne of 2 routes

1. L-histidine + H(+) = histamine + CO2

Catalysed by HDC

1. L-histidine(out) + L-arginine(in) + Na(+)(out) = L-histidine(in) + L-arginine(out) + Na(+)(in)
2. N(omega)-methyl-L-arginine(in) + L-arginine(out) = N(omega)-methyl-L-arginine(out) + L-arginine(in)

Catalysed by SLC7A6, SLC25A29inner side of the membrane, outer side of the membraneOne of 2 routes

1. L-histidine + H(+) = histamine + CO2
2. transport of histamine (cytosol to extracellular)

Catalysed by HDC, SLCO1A2, SLCO1B3extracellular space, cytosolOne of 2 routes