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Thermal Stability of Malic Dehydrogenase from Heat-Hardened Phaseolus acutifolius ‘Tepary Buff’¹

Malic dehydrogenase was extracted from high temperature hardened and unhardened leaves of 4- to 5-weekold, greenhouse grown, 'Tepary Buff beans. Purified fractions of malic dehydrogenase were exposed to temperatures that ranged from 30 to 62 C for 15 minutes. The specific activity of the enzyme was markedly reduced by temperatures above 50 C. Malic dehydrogenase extracted from hardened plants was significantly more thermostable than the extracts from unhardened tissue. The extracts from hardened tissue also exhibited a significantly higher specific activity over a wider pH range than the extracts from unhardened tissue.

The results from heat tolerance tests with intact leaf discs correlated well with the reduction in activity of the extracted malic dehydrogenase. These data indicate that enzyme denaturation is an important factor in heat tolerance. One way that high temperature hardening may increase the heat tolerance of higher plants is by increasing the thermal stability of enzymes.

² Associate Professor, Assistant Professor, and Research Assistant, Department of Horticulture and Forestry, University of Nebraska, Lincoln, Nebr.

Received for publication October 13, 1966.