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Enhancement of Lactase Activity in Milk by Reactive Sulfhydryl Groups Induced by Heat Treatment

J. Jiménez-Guzmán^*, A. E. Cruz-Guerrero^*, G. Rodríguez-Serrano^*, A. López-Munguía, L Gómez-Ruiz^* and M García-Garibay^*

^* Departamento de Biotecnología, Universidad Autónoma Metropolitana, Iztapalapa, A.P. 55-535, Mexico City D.F., 09340 Mexico.
Instituto de Biotecnología, Universidad Nacional Autónoma de México, A.P. 510-3, Cuernavaca, Mor., 62271, México

Corresponding Author:
Dr. Mariano García-Garibay; e-mail:
jmgg{at}xanum.uam.mx.

The effects of heat treatments of milk and whey prior to lactose hydrolysis with Kluyveromyces lactis ß-galactosidase were studied. It was observed that heat treatment of milk significantly increases lactase activity, with a maximum activity increase found when milk was heated at 55°C. In whey from 55 up to 75°C, ß-galactosidase activity decreased slightly. Nevertheless, heating whey at 85°C for 30 min raised the rate of hydrolysis significantly. Electrophoretic patterns and UV spectra proved that the activity change correlated with milk protein denaturation, particularly that of ß-lactoglobulin. Heating whey permeate did not increase the enzyme activity as heating whole whey; but heating whey prior to ultrafiltration also resulted in enzyme activation. Measurement of free sulfhydryl (SH) groups in both whey and heated whey permeate showed that the liberation of free SH is highly correlated to the change of the activity. Furthermore, this activation can be reversed by oxidizing the reactive sulfhydryl groups, proving that the observed effect may be related to the release of free SH to the medium, rather than to the denaturation of a thermolabile protein inhibitor.

Key Words: ß-galactosidase • lactase • sulfhydryl groups • whey protein

Abbreviation key: ONP= ortho-nitro-phenol, ONPG= ortho-nitro-phenyl-ß-D-galactoside, Rf= migration coefficient, SH= sulfhydryl, ₀= hydrolysis rate

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