Final answer:
The enzyme DNase, which specifically degraded DNA, was the one that destroyed the transforming principle in the Avery-MacLeod-McCarty experiments, leading to the conclusion that DNA was the hereditary material.
Step-by-step explanation:
The enzymes that destroyed the transforming principle in the experiments conducted by Avery and his colleagues were DNase, which specifically degraded DNA. In their systematic study, Avery, MacLeod, and McCarty were able to show that only when DNA was degraded by DNase, the mixture failed to transform the R strain of bacteria. All other mixtures, where proteins and RNA were degraded, still managed to transform the R strain, leading the scientists to conclude that DNA was the substance responsible for transformation, which meant that DNA was the hereditary material, the "stuff of genes".