Defects in enzyme regulation versus defects in enzyme synthesis as cause of metabolic disorders.

Based on the consideration that normal metabolic processes depend upon the activity of key enzymes (and membrane carriers) as modulated by regulatory factors (hormones, diet, endogenous compounds, age, physical activity, environmental agents), metabolic disorders might be classified into two groups: (I) defects in enzyme synthesis, leading to enzyme deficiency (classical inborn errors of metabolism) or to qualitative (structural) enzyme alterations (entailing unresponsiveness to regulation), in the presence of normal regulatory factors; (II) defects in enzyme regulation, which include metabolic syndromes such as diabetes mellitus, obesity and hyperlipoproteinemias (other than type I), and are due to changes in enzyme activities caused by alterations in regulatory factor(s) (secondary to various causes), in the presence of normally responsive enzymes.