An oncological patient was prescribed methotrexate. With the lapse of time target cells of the tumour lost susceptibility to this drug. There is change of gene expression of the folowing enzyme:

Answer options

1. Deaminase
2. Folate oxidase
3. Thiaminase
4. Dehydrofolate reductase
5. Folate decarboxylase

Explanation

Methotrexate is structurally related to folic acid and acts as an antagonist of that vitamin by inhibiting dihydrofolate reductase in humans, which is the enzyme that converts folic acid to its active coenzyme form, tetrahydro folic acid.