All living organisms rely on vitamin B6 for their survival, as it is an essential cofactor in hundreds of enzymatic reactions. Vitamin B6 refers to a family of chemical compounds that can be interconverted enzymatically in a salvage pathway. The main focus of this work is on the salvage pathway enzyme PDX3 of the model plant Arabidopsis thaliana. In plants, PDX3 consists of two domains: an N-terminal epimerase domain that is involved in a repair pathway for the co-factors nicotinamide adenine dinucleotide (NADH) and its phosphate NADPH and a C-terminal oxidase domain taking part in vitamin B6 salvage. Both domains were functionally characterized in this thesis. NAD(P)H repair involves the concomitant action of the N-terminal domain and an independent enzyme and is dispensable for normal plant development. The absence of PDX3 leads to an imbalance of the different vitamin B6 forms, aberrant growth and development and an impaired nitrogen metabolism.