What characteristic defines a methylated compound within biological systems?

A methylated compound is characterized by the addition of a methyl group (CH3) to a molecule, which is a process known as methylation. In biological systems, one of the most significant roles of methylated compounds is their function in epigenetic regulation. This process involves the modification of DNA and histones, affecting gene expression without altering the underlying DNA sequence. Methylation can lead to the silencing of genes or the regulation of transcription, impacting various biological processes such as development, differentiation, and responses to environmental stimuli.

The role of methylation in epigenetics is crucial for the proper functioning of cells, influencing everything from cell identity to the potential development of diseases like cancer. Hence, the correct answer highlights the importance of methylated compounds, particularly in how they modulate genetic activity.

The other options do not align with the defining characteristics of methylated compounds. For instance, methylated compounds are not necessarily hydrophilic, and they do not automatically exhibit enhanced reactivity; in fact, methylation can reduce reactivity for some functional groups. Additionally, while methylated compounds can participate in various metabolic pathways, they certainly play a critical role in cellular metabolism but are not defined by an inability to do so.