CONVERSION OF ALL-TRANS BETA-CAROTENE TO RETINAL BY AN ENZYME FROM THE INTESTINAL-MUCOSA OF HUMAN NEONATES

The enzymatic conversion of all trans beta-carotene to retinal by an i ntestinal mucosal enzyme (beta-carotene cleavage enzyme, BCC) from aut opsy samples of human neonates was demonstrated for the first time. Th e enzymatic product was characterized as its O-ethyl oxime, which, on high pressure liquid chromatography (HPLC), yielded a sharp peak corre sponding to an authentic retinal (O-ethyl) oxime. The enzyme blank and boiled enzyme blank failed to show any significant HPLC peaks corresp onding to retinal (O-ethyl) oxime, retinal, or retinol. Based on the o bserved activities among intestinal samples from 14 premature infants, the BCC enzyme activity ranged from 3.3-1210 pmoles per mg mucosal pr otein 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 a ctivity of the fresh intestine is lost in storage of the dead animal f or 8 hr at 25-degrees-C followed by storage at 4-degrees-C for 16 hr. More importantly, 70% of the fresh enzyme activity is lost after stora ge of the animals at 4-degrees-C for only 8 hr. Thus, the observed act ivities in the human autopsy samples appear to be markedly underestima ted because of the marked loss of enzyme activity from the time of dea th to the time of assay. Therefore, the true activity of the enzyme ca n be assessed only after the extent of loss of activity on storage of the human samples can be accurately measured. In spite of repeated att empts, no detectable BCC activity was found in the placentas of pre-te rm or term infants. Nonetheless, the demonstration of BCC enzyme activ ity in the intestinal mucosa of human neonates shows that beta-caroten e can serve as an important source of vitamin A in newborn infants.