Conversion of all trans β-carotene to retinal by an enzyme from the intestinal mucosa of human neonates

Abstract

The enzymatic conversion of all trans β-carotene to retinal by an intestinal mucosal enzyme (β-carotene cleavage enzyme, BCC) from autopsy samples of human neonates was demonstrated for the first time. The enzymatic product was characterized as its O-ethyl oxime, which, on high pressure liquid chromatography (HPLC), yielded a sharp peak corresponding to an authentic retinal ( O-ethyl) oxime. The enzyme blank and boiled enzyme blank failed to show any significant HPLC peaks corresponding to retinal ( O-ethyl) oxime, retinal, or retinol. Based on the observed activities among intestinal samples from 14 premature infants, the BCC enzyme activity ranged from 3.3–1210 pmoles per mg mucosal protein per hr. Studies on the stability of the enzyme using the rat as the experimental animal revealed that as much as 80% of the original activity of the fresh intestine is lost in storage of the dead animal for 8 hr at 25° C followed by storage at 4° C for 16 hr. More importantly, 70% of the fresh enzyme activity is lost after storage of the animals at 4° C for only 8 hr. Thus, the observed activities in the human autopsy samples appear to be markedly underestimated because of the marked loss of enzyme activity from the time of death to the time of assay. Therefore, the true activity of the enzyme can be assessed only after the extent of loss of activity on storage of the human samples can be accurately measured. In spite of repeated attempts, no detectable BCC activity was found in the placentas of pre-term or term infants. Nonetheless, the demonstration of BCC enzyme activity in the intestinal mucosa of human neonates shows that β-carotene can serve as an important source of vitamin A in newborn infants.