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2-Hydroxy-4-(methylthio)butanoic Acid Is a Naturally Occurring Methionine Precursor in the Chick

Monsanto Company, Animals Sciences Division, Chesterfield, MO 63198

The objective of these experiments was to determine the origin of 2-hydroxy-4-(methylthio)butanoic acid (HMB) detected in the liver and excreta of chicks that had never been fed Alimet® (an 88% aqueous solution of HMB) or MHA® (an 86% calcium salt of HMB). Gas chromatography and mass spectrometry were used to identify HMB in these birds. A normal biochemical pathway of 5'-deoxy-5'-methylthioadenosine (MTA) is proposed as the source of naturally occurring HMB. Studies of conversion of [methyl-¹⁴C]MTA to L-methionine by chick liver enzymes showed that radiolabeled HMB and 2-oxo-4-(methylthio)butanoic acid (keto-methionine) were synthesized during the reaction. Specific radioactivities of labeled HMB and keto-methionine showed that HMB is not synthesized from keto-methionine. Fractionation studies indicated that radiolabeled HMB formed from [¹⁴C]MTA could be used in the synthesis of L-methionine in the presence of peroxisomes and/or mitochondrial enzymes. In this way, HMB synthesized from MTA by chick liver enzymes functions as a precursor of L-methionine and as an intermediate in a naturally occurring pathway for the synthesis of L-methionine in the chick.

KEY WORDS: • 5'-deoxy-5'-methylthioadenosine • methionine • Alimet • chicks • 2-hydroxy-4-(methylthio)butanoic acid

Manuscript received 14 April 1989. Revision accepted 16 January 1990.

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