Why does ribonuclease become inactive when treated with mercaptoethanol?

Ribonuclease becomes inactive when treated with mercaptoethanol because mercaptoethanol reduces disulfide bonds in the protein, which are crucial for maintaining its three-dimensional structure. These disulfide bonds stabilize the enzyme's active conformation; when they are disrupted, the enzyme unfolds or misfolds, leading to a loss of its catalytic activity. Without the proper structure, ribonuclease cannot effectively bind to its RNA substrates.