1. The efficiencies of three different methods of inhibiting fatty acid production in chyme have been evaluated. The effects of each method on the phase distribution of fatty acids after ultracentrifugation have been studied.

2. Chyme fatty acid concentrations were measured during a 4 h incubation (a) without an inhibitor, (b) after acid treatment, (c) after addition of p-bromophenyl-boronic acid and (d) after heating.

3. In a separate experiment, fresh chyme was incubated to allow equilibration of lipolysis. Aliquots were treated by each inactivation method and ultracentrifuged overnight to separate the phases. Total and micellar fatty acids and glycerides were measured.

4. In the first experiment, acid treatment completely inhibited fatty acid generation producing 4 h concentrations which were 93.4 ± 5.6% (mean ± sem) of initial values compared with 398.0 ± 54.0% (P < 0.05) for uninhibited samples. p-Bromophenylboronic acid and heating gave significant but incomplete inhibition (132.9 ± 6.6% and 166.1 ± 15.2% of initial concentrations, respectively).

5. Ultracentrifugation disclosed five phases in all except the acid-treated samples, which had four. Micellar phase fatty acid concentrations were significantly higher in the acid-treated than in the untreated samples (2.6 ± 0.7 versus 1.7 ± 0.5 mmol/l, P = 0.05), as were glyceride concentrations (1.5 ± 0.4 vs 0.6 ± 0.3 mmol/l, P = 0.05).

6. It was concluded that acid treatment was the most efficient inhibitor of fatty acid production, but it disrupted the phases. p-Bromophenylboronic acid gave significant inhibition without causing phase disruption and was therefore the most useful inhibitor overall.

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