Ch Ch

Figure 28.17 • Menaquinone general structure. Menaquinone-4 (n = 4) and menaquinone-6 (n = 6) are the most common forms.

overall dietary intake is unresolved110 but these help explain the high level of vitamin K activity in feces.

Following absorption by the enterocytes, and similar to other dietary fats, phylloquinone and menaquinones are found in the newly formed chylomicrons, which are released into the lymph and bloodstream. The primary form of circulating vitamin K activity is phylloquinone. Conversion of the chylomicrons into chylomicron remnants and their subsequent association with apolipoprotein E result in receptor-mediated uptake by cells through pinocytosis. Historically, phylloquinone uptake by hepatic cells was thought to be the predominant process; however, it is now recognized that uptake by osteoblast precursor cells also occurs and explains the ultimate delivery to osteoblasts inside bone tissue.111 Approximately 90% of the vitamin K found in the liver is in the form of menaquinones and the remainder is phyllo-quinone.110 In contrast to other fat-soluble vitamins, no significant storage occurs. The major urinary metabolites are glucuronide conjugates of carboxylic acids derived from shortening of the side chain.

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