Whereas
all forms of vitamin K appear to be initially associated
with triglyceride-rich lipoproteins (TRL), the longer
chain menaquinones including MK-7 and MK-9 are
also associated with low-density lipoprotein (LDL).
MK-4 has been reported in TRL, LDL, and high density
lipoproteins. These preliminary data suggest that the
menaquinones have different transport pathways and
distribution, which has implications for transport to
extra-hepatic tissue such as bone (33).
Emerging studies on MK-4 challenge our current
understanding of vitamin K. As demonstrated using
stable isotopes, MK-4 is a conversion product of phylloquinone
via the intermediate, menadione (37). There
appears to be both local and systemic conversion to
MK-4, with the local conversion being the predominant