1483 HUMAN MILK LIPASE ASSOCIATED WITH BREAST MILK JAUNDICE: SUBSTRATE SPECIFICITY

Abstract

Human milk samples that inhibit bilirubin-UDP-glucuronyl transferase (UDPGT) activity in vitro are associated with the syndrome of breast milk jaundice. We have reported that the inhibitor appears to be non-esterified fatty acid and the milk samples associated with the clinical syndrome exhibit significantly higher unstimulated lipase activity (P<0.01) and specific activity (P<0.001) than controls using a pH-stat assay with tributyrin as substrate. (Pediatr. Res. 14:1328, 1981).

The present study was done to test whether that lipolytic activity was due to a specific lipase or to a non-specific esterase. Two water soluble substrates were tested: p-nitrophenyl (p-NP) acetate (for non-specific esterase) and p-NP butyrate (a fatty acyl ester). Assays with these substrates were performed simultaneously along with the above tributrin assay using 23 human milk samples as sources of enzyme. Enzyme units are defined as 1 μM of substrate released by 1 ml of milk per minute. Lipase activity (tributrin assay) ranged from 3.1 to 11.1 units. Activity vs. p-NP butyrate (fatty acyl ester) ranged from 1.1 to 23.7 units and activity vs. p-NP acetate (non-specific ester) ranged from 0.3 to 2.6 units. Tributyrin assay activity correlated positively with p-NP butyrate activity (r=0.66, P<0.001) but did not correlate with p-NP acetate activity (r=0.07, N.S.). These results are consistent with the hypothesis that the unstimulated enzyme activity we described associatedwith breast milk jaundice is produced by a lipase and not a non-specific esterase.

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