An isolated nucleotide sequence or fragment thereof encoding the porcine intrinsic factor, wherein the porcine intrinsic factor comprises an amino acid sequence having at least 85% amino acid sequence identity to an amino acid sequence selected from the group consisting of SEQ ID NO: 3, SEQ ID NO: 6, and SEQ ID NO: 9. The invention also encompasses an isolated nucleic acid sequence or fragment thereof comprising, or complementary to, a nucleotide sequence having at least 85% nucleotide sequence identity to a nucleotide sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 4, and SEQ ID NO: 7. The porcine intrinsic factor can be use is an assay to determine the quantity of vitamin B12 in a biological sample.