Why is cysteine considered essential in preterm infants?

• A. Because the enzyme necessary for cysteine synthesis via the methionine trans-sulfuration pathway is not present in adequate amounts in preterm infants
• B. Because cysteine is not required by preterm infants due to maternal supply
• C. Because cysteine can be synthesized from methionine in preterm infants but is insufficient
• D. Because cysteine deficiency is a universal newborn deficiency

Answer: (A)

Cysteine is considered essential in preterm infants because their ability to make it endogenously is limited by immature trans-sulfuration enzymes. The pathway that converts methionine to cysteine relies on enzymes such as cystathionine beta-synthatase and cystathionine gamma-lyase, whose activity is not yet adequate in preterm infants, so cysteine production from methionine is insufficient to meet needs. As a result, preterm infants depend on dietary cysteine to support critical functions like glutathione synthesis and antioxidant protection. Maternal supply cannot fully compensate postnatally, and while some synthesis from methionine occurs, it is not enough, making cysteine effectively essential for this group.