The secret to sushi's delicious taste is invisible to the human eye. Chefs spend years training to properly prepare the Japanese culinary staple, which consists of fresh fish and seasoned rice, either served together or wrapped in seaweed. At its most elemental, as the American Chemistry Society's latest Reactions video explains below, the bite-sized morsels contain an assortment of compounds that, together, combine to form a perfectly balanced mix of savory and sweet. They include mannitol, iodine, and bromophenol, all of which provide a distinctive tang; and glutamate, which adds a savory, rich umami flavor (and turns into MSG when it's combined with a sodium ion).
In circulation, cortisol can be found in free form, but the majority is bound to either liver-derived corticosteroid-binding globulin (CBG) or albumin due to its lipophilic nature. The normal plasma level of CBG is a relatively constant 40 mg/L ( μmol), which binds about 70% of plasma cortisol(14 μg/dL). However, plasma CBG levels are dynamically regulated. During pregnancy, plasma levels of CBG rise dramatically, while acute stresses, such as burn injury or sepsis, can lead to dramatic decreases in plasma CBG. Cortisol binds to CBG with high affinity (kDa = × 10 -7 M; half-life of steroid binding = 5 days). Other steroids, including progesterone, prednisolone, and aldosterone, compete for binding sites on CBG, and high levels of one steroid will displace the others. For example, therapeutic levels of prednisone displace 35% of CBG-bound cortisol. In contrast, many synthetic glucocorticoids, including dexamethasone, fail to bind CBG. Albumin circulates in plasma at a concentration of 40 g/L ( to mmol) and has low affinity for cortisol (kDa 5 × 10 -5 M). Only 20% of plasma cortisol is bound to albumin. At low serum cortisol levels, most of the cortisol is bound to CBG. However, the binding capacity of CBG is saturated at a cortisol concentration of 28 μg/dL, a level that is frequently exceeded in stressed patients. With elevations in plasma cortisol, there is an increased proportion of albumin-bound and free cortisol, whereas the amount of CBG-bound cortisol remains the same. The concentration of free cortisol is 1 μg/dL at a normal total plasma cortisol of 20 μg/dL, although this value can rise to as high as 15 to 50 μg/dL after ACTH stimulation. Free cortisol is considered to be the active form because protein-bound cortisol cannot easily pass through cell membranes. As a result of rapid equilibration between the bound and free fractions of cortisol, the bound fraction acts as a reservoir. Thus, increases in plasma CBG such as occur during pregnancy can greatly increase the amount of cortisol reserves available at a constant rate of synthesis, whereas short-term decreases in plasma CBG during acute stress can significantly increase the pool of free cortisol. That total plasma cortisol falls below 5 μg/dL at night exemplifies how rapidly cortisol can leave the plasma. Aldosterone does not have a specific binding protein, but binds weakly to albumin. A normal plasma aldosterone level is μg/dL ( nmol). Other steroids with mineralocorticoid activity, such as corticosterone and 11-deoxycorticosterone, do bind CBG. CBG-bound corticosteroids are resistant to metabolism.